gh-143535: Document dispatch behavior of singledispatchmethod

Doc/library/functools.rst

@@ -692,6 +692,11 @@
         @neg.register
         def _(self, arg: bool):
             return not arg
+   .. 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``.
 
    ``@singledispatchmethod`` supports nesting with other decorators such as
    :deco:`classmethod`. Note that to allow for