Rewrite the documentation for most of anaphoric

Arguably, it's still not great. It's perhaps more terse than would be helpful. My goal for now is just to make sure that it's correct, and that it doesn't mislead with e.g. suggestions that these macros support only true lists, instead of arbitrary iterable objects.

docs/extra/anaphoric.rst

@@ -28,10 +28,16 @@ Stephen King book is" 'it)``.
 ap-if
 =====
 
-Usage: ``(ap-if (foo) (print it))``
+Usage: ``(ap-if test-form then-form else-form)``
 
-Evaluates the first form for truthiness, and bind it to ``it`` in both the
-true and false branches.
+As :ref:`if <if>`, but the result of the test form is named ``it`` in
+the subsequent forms. As with ``if``, the else-clause is optional.
+
+.. code-block:: hy
+
+   => (import os)
+   => (ap-if (.get os.environ "PYTHONPATH")
+   ...  (print "Your PYTHONPATH is" it))
 
 
 .. _ap-each:
@@ -39,9 +45,17 @@ true and false branches.
 ap-each
 =======
 
-Usage: ``(ap-each [1 2 3 4 5] (print it))``
+Usage: ``(ap-each xs body…)``
 
-Evaluate the form for each element in the list for side-effects.
+Evaluate the body forms for each element ``it`` of ``xs`` and return
+``None``.
+
+.. code-block:: hy
+
+   => (ap-each [1 2 3] (print it))
+   1
+   2
+   3
 
 
 .. _ap-each-while:
@@ -49,10 +63,10 @@ Evaluate the form for each element in the list for side-effects.
 ap-each-while
 =============
 
-Usage: ``(ap-each-while list pred body)``
+Usage: ``(ap-each-while xs pred body…)``
 
-Evaluate the form for each element where the predicate form returns
-``True``.
+As ``ap-each``, but the form ``pred`` is run before the body forms on
+each iteration, and the loop ends if ``pred`` is false.
 
 .. code-block:: hy
 
@@ -66,11 +80,10 @@ Evaluate the form for each element where the predicate form returns
 ap-map
 ======
 
-Usage: ``(ap-map form list)``
+Usage: ``(ap-map form xs)``
 
-The anaphoric form of map works just like regular map except that
-instead of a function object it takes a Hy form. The special name
-``it`` is bound to the current object from the list in the iteration.
+Create a generator like :py:func:`map` that yields each result of ``form``
+evaluated with ``it`` bound to successive elements of ``xs``.
 
 .. code-block:: hy
 
@@ -83,10 +96,12 @@ instead of a function object it takes a Hy form. The special name
 ap-map-when
 ===========
 
-Usage: ``(ap-map-when predfn rep list)``
+Usage: ``(ap-map-when predfn rep xs)``
 
-Evaluate a mapping over the list using a predicate function to
-determin when to apply the form.
+As ``ap-map``, but the predicate function ``predfn`` (yes, that's a
+function, not an anaphoric form) is applied to each ``it``, and the
+anaphoric mapping form ``rep`` is only applied if the predicate is true.
+Otherwise, ``it`` is yielded unchanged.
 
 .. code-block:: hy
 
@@ -102,11 +117,9 @@ determin when to apply the form.
 ap-filter
 =========
 
-Usage: ``(ap-filter form list)``
+Usage: ``(ap-filter form xs)``
 
-As with ``ap-map`` we take a special form instead of a function to
-filter the elements of the list. The special name ``it`` is bound to
-the current element in the iteration.
+The :py:func:`filter` equivalent of ``ap-map``.
 
 .. code-block:: hy
 
@@ -119,11 +132,9 @@ the current element in the iteration.
 ap-reject
 =========
 
-Usage: ``(ap-reject form list)``
+Usage: ``(ap-reject form xs)``
 
-This function does the opposite of ``ap-filter``, it rejects the
-elements passing the predicate . The special name ``it`` is bound to
-the current element in the iteration.
+Equivalent to ``(ap-filter (not form) xs)``.
 
 .. code-block:: hy
 
@@ -136,10 +147,9 @@ the current element in the iteration.
 ap-dotimes
 ==========
 
-Usage ``(ap-dotimes n body)``
+Usage: ``(ap-dotimes n body…)``
 
-This function evaluates the body *n* times, with the special
-variable ``it`` bound from *0* to *1-n*. It is useful for side-effects.
+Equivalent to ``(ap-each (range n) body…)``.
 
 .. code-block:: hy
 
@@ -154,15 +164,15 @@ variable ``it`` bound from *0* to *1-n*. It is useful for side-effects.
 ap-first
 ========
 
-Usage ``(ap-first predfn list)``
+Usage: ``(ap-first form xs)``
 
-This function returns the first element that passes the predicate or
-``None``, with the special variable ``it`` bound to the current element in
-iteration.
+Evaluate the predicate ``form`` for each element ``it`` of ``xs``. When
+the predicate is true, stop and return ``it``. If the predicate is never
+true, return ``None``.
 
 .. code-block:: hy
 
-   =>(ap-first (> it 5) (range 10))
+   => (ap-first (> it 5) (range 10))
    6
 
 
@@ -171,15 +181,15 @@ iteration.
 ap-last
 ========
 
-Usage ``(ap-last predfn list)``
+Usage: ``(ap-last form list)``
 
-This function returns the last element that passes the predicate or
-``None``, with the special variable ``it`` bound to the current element in
-iteration.
+Evaluate the predicate ``form`` for every element ``it`` of ``xs``.
+Return the last element for which the predicate is true, or ``None`` if
+there is no such element.
 
 .. code-block:: hy
 
-   =>(ap-last (> it 5) (range 10))
+   => (ap-last (> it 5) (range 10))
    9
 
 
@@ -188,18 +198,24 @@ iteration.
 ap-reduce
 =========
 
-Usage ``(ap-reduce form list &optional initial-value)``
+Usage: ``(ap-reduce form xs &optional initial-value)``
 
-This function returns the result of applying form to the first 2
-elements in the body and applying the result and the 3rd element
-etc. until the list is exhausted. Optionally an initial value can be
-supplied so the function will be applied to initial value and the
-first element instead. This exposes the element being iterated as
-``it`` and the current accumulated value as ``acc``.
+This macro is an anaphoric version of :py:func:`reduce`. It works as
+follows:
+
+- Bind ``acc`` to the first element of ``xs``, bind ``it`` to the
+  second, and evaluate ``form``.
+- Bind ``acc`` to the result, bind ``it`` to the third value of ``xs``,
+  and evaluate ``form`` again.
+- Bind ``acc`` to the result, and continue until ``xs`` is exhausted.
+
+If ``initial-value`` is supplied, the process instead begins with
+``acc`` set to ``initial-value`` and ``it`` set to the first element of
+``xs``.
 
 .. code-block:: hy
 
-   =>(ap-reduce (+ it acc) (range 10))
+   => (ap-reduce (+ it acc) (range 10))
    45
 
 
@@ -208,7 +224,7 @@ first element instead. This exposes the element being iterated as
 #%
 ==
 
-Usage ``#% expr``
+Usage: ``#% expr``
 
 Makes an expression into a function with an implicit ``%`` parameter list.