[3.13] gh-140873: Fix the singledispatchmethod documentation (GH-141523) (GH-141710)

It does not support non-descriptor callables yet.
(cherry picked from commit d89eb2f984)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

Doc/library/functools.rst

@@ -620,7 +620,7 @@ The :mod:`functools` module defines the following functions:
    dispatch>` :term:`generic function`.
 
    To define a generic method, decorate it with the ``@singledispatchmethod``
-   decorator. When defining a function using ``@singledispatchmethod``, note
+   decorator. When defining a method using ``@singledispatchmethod``, note
    that the dispatch happens on the type of the first non-*self* or non-*cls*
    argument::
 

Lib/functools.py

@@ -947,8 +947,7 @@ def singledispatch(func):
 class singledispatchmethod:
     """Single-dispatch generic method descriptor.
 
-    Supports wrapping existing descriptors and handles non-descriptor
-    callables as instance methods.
+    Supports wrapping existing descriptors.
     """
 
     def __init__(self, func):