pythongh-95276: Add callable entry to the glossary

Author: MonadChains

Doc/glossary.rst

@@ -203,6 +203,12 @@ Glossary
       A list of bytecode instructions can be found in the documentation for
       :ref:`the dis module <bytecodes>`.
 
+   callable
+      A callable is an object that has the pointer :c:member:`~PyTypeObject.tp_call`
+      not ``NULL`` and set to a function. Python functions and object methods
+      are callables. An instance of a class that implements the :meth:`~object.__call__`
+      method is also a callable.
+
    callback
       A subroutine function which is passed as an argument to be executed at
       some point in the future.