gh-143535: Document dispatch behavior of singledispatchmethod

Author: priyanshu2282-cyber

Doc/library/functools.rst

@@ -679,7 +679,11 @@ The :mod:`functools` module defines the following functions:
    decorator. When defining a method using ``@singledispatchmethod``, note
    that the dispatch happens on the type of the first non-*self* or non-*cls*
    argument::
-
+   .. note::
+   ``singledispatchmethod`` dispatches the first argument during call
+   time irrespective of whether the method is bound or unbound, as a result,
+   ``x.f(a) != C.f(x, a)``.The behavior is intentional and required to support correct dispatch for
+   ``staticmethod`` and ``classmethod``.
     class Negator:
         @singledispatchmethod
         def neg(self, arg):