bpo-32500: Correct the documentation for PySequence_Size() and PySequence_Length() (GH-5767)

Dropped the part that says: "For objects that do not provide sequence protocol".
(cherry picked from commit 7a1e1786)
Co-authored-by: Zackery Spytz <zspytz@gmail.com>

Doc/c-api/sequence.rst

@@ -17,9 +17,8 @@ Sequence Protocol
 
    .. index:: builtin: len
 
-   Returns the number of objects in sequence *o* on success, and ``-1`` on failure.
-   For objects that do not provide sequence protocol, this is equivalent to the
-   Python expression ``len(o)``.
+   Returns the number of objects in sequence *o* on success, and ``-1`` on
+   failure.  This is equivalent to the Python expression ``len(o)``.
 
    .. versionchanged:: 2.5
       These functions returned an :c:type:`int` type. This might require