bpo-25866: Minor cleanups to "sequence" in docs (GH-17177)

Author: alclarks

Doc/reference/datamodel.rst

@@ -1566,7 +1566,7 @@ not found on a module object through the normal lookup, i.e.
 the module ``__dict__`` before raising an :exc:`AttributeError`. If found,
 it is called with the attribute name and the result is returned.
 
-The ``__dir__`` function should accept no arguments, and return a list of
+The ``__dir__`` function should accept no arguments, and return a sequence of
 strings that represents the names accessible on module. If present, this
 function overrides the standard :func:`dir` search on a module.
 
@@ -1797,6 +1797,10 @@ Notes on using *__slots__*
   (the other bases must have empty slot layouts) - violations raise
   :exc:`TypeError`.
 
+* If an iterator is used for *__slots__* then a descriptor is created for each
+  of the iterator's values. However, the *__slots__* attribute will be an empty
+  iterator.
+
 .. _class-customization:
 
 Customizing class creation
@@ -2128,8 +2132,8 @@ operators.  It is recommended that both mappings and sequences implement the
 mappings, ``in`` should search the mapping's keys; for sequences, it should
 search through the values.  It is further recommended that both mappings and
 sequences implement the :meth:`__iter__` method to allow efficient iteration
-through the container; for mappings, :meth:`__iter__` should be the same as
-:meth:`keys`; for sequences, it should iterate through the values.
+through the container; for mappings, :meth:`__iter__` should iterate
+through the object's keys; for sequences, it should iterate through the values.
 
 .. method:: object.__len__(self)
 
@@ -2243,9 +2247,9 @@ through the container; for mappings, :meth:`__iter__` should be the same as
 
 
 The membership test operators (:keyword:`in` and :keyword:`not in`) are normally
-implemented as an iteration through a sequence.  However, container objects can
+implemented as an iteration through a container. However, container objects can
 supply the following special method with a more efficient implementation, which
-also does not require the object be a sequence.
+also does not require the object be iterable.
 
 .. method:: object.__contains__(self, item)