gh-143535: Document dispatch behavior of singledispatchmethod

Doc/library/functools.rst

@@ -718,6 +718,12 @@ The :mod:`functools` module defines the following functions:
    The same pattern can be used for other similar decorators:
    :deco:`staticmethod`, :deco:`~abc.abstractmethod`, and others.
 
+    ``singledispatchmethod`` always dispatches on the first argument passed at call
+    time, irrespective of whether the method is bound or unbound. As a result,
+    calling the method through an instance or through the class may result in
+    different dispatch behavior. This behavior is required to support correct
+    dispatch when used with ``staticmethod`` and ``classmethod``.
+
    .. versionadded:: 3.8
 
    .. versionchanged:: 3.15