Non-nil if FUNCTION makes provisions for interactive calling.
This means it contains a description for how to read arguments to give it.
The value is nil for an invalid function or a symbol with no function
Interactively callable functions include strings and vectors (treated
as keyboard macros), lambda-expressions that contain a top-level call
to `interactive', autoload definitions made by `autoload' with non-nil
fourth argument, and some of the built-in functions of Lisp.
Also, a symbol satisfies `commandp' if its function definition does so.
If the optional argument FOR-CALL-INTERACTIVELY is non-nil,
then strings and vectors are not accepted. (fn FUNCTION &optional FOR-CALL-INTERACTIVELY)