gh-143535: Document dispatch behavior of singledispatchmethod

Doc/library/functools.rst

@@ -718,6 +718,11 @@ 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.
 
+   For every method decorated with ``@singledispatchmethod`` in a class, the value
+   dispatched on by that method is *always* the first argument passed for the call.
+   Therefore, dispatching to regular methods can only be sensibly performed by
+   calling such a method from instances of the class, and not from the class object.
+
    .. versionadded:: 3.8
 
    .. versionchanged:: 3.15