[3.14] pythongh-140873: Fix the singledispatchmethod documentation (pythonGH-141523)

It does not support non-descriptor callables yet.

Author: serhiy-storchaka

Doc/library/functools.rst

@@ -672,7 +672,7 @@ 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

@@ -995,8 +995,7 @@ def wrapper(*args, **kw):
 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):