A few documentation improvements, spurred on by Brett's review.

Doc/library/sys.rst

@@ -619,12 +619,13 @@ always available.
 
 .. data:: implementation
 
-   An object containing the information about the implementation of the
-   currently running Python interpreter.  Its attributes are the those
-   that all Python implementations must implement.  They are described
-   below.
+   An object containing information about the implementation of the
+   currently running Python interpreter.  The following attributes are
+   required to exist in all Python implementations.
 
-   *name* is the implementation's identifier, like ``'cpython'``.
+   *name* is the implementation's identifier, e.g. ``'cpython'``.  The actual
+   string is defined by the Python implementation, but it is guaranteed to be
+   lower case.
 
    *version* is a named tuple, in the same format as
    :data:`sys.version_info`.  It represents the version of the Python
@@ -633,7 +634,7 @@ always available.
    interpreter conforms, which ``sys.version_info`` represents.  For
    example, for PyPy 1.8 ``sys.implementation.version`` might be
    ``sys.version_info(1, 8, 0, 'final', 0)``, whereas ``sys.version_info``
-   would be ``sys.version_info(1, 8, 0, 'final', 0)``.  For CPython they
+   would be ``sys.version_info(2, 7, 2, 'final', 0)``.  For CPython they
    are the same value, since it is the reference implementation.
 
    *hexversion* is the implementation version in hexadecimal format, like
@@ -646,10 +647,12 @@ always available.
    ``cache_tag`` is set to ``None``, it indicates that module caching should
    be disabled.
 
-   Regardless of its contents, :data:`sys.implementation` will not
-   change during a run of the interpreter, nor between implementation
-   versions.  (It may change between Python language versions,
-   however.)  See `PEP 421` for more information.
+   :data:`sys.implementation` may contain additional attributes specific to
+   the Python implementation.  These non-standard attributes must start with
+   an underscore, and are not described here.  Regardless of its contents,
+   :data:`sys.implementation` will not change during a run of the interpreter,
+   nor between implementation versions.  (It may change between Python
+   language versions, however.)  See `PEP 421` for more information.
 
    .. versionadded:: 3.3