Fix inspect.ismethod() doc

Closes #16851: Fix inspect.ismethod() doc, return also True if object is an
unbound method.

Patch written by Anna Koroliuk.

Doc/library/inspect.rst

@@ -272,7 +272,8 @@ Note:
 
 .. function:: ismethod(object)
 
-   Return true if the object is a bound method written in Python.
+   Return true if the object is a bound or unbound method written in Python.
+
 
 
 .. function:: isfunction(object)