Insert the list of all defined keys and their definitions.
The list is inserted in the current buffer, while the bindings are
looked up in BUFFER.
The optional argument PREFIX, if non-nil, should be a key sequence;
then we display only bindings that start with that prefix.
The optional argument MENUS, if non-nil, says to mention menu bindings.
(Ordinarily these are omitted from the output.)
(fn BUFFER &optional PREFIX MENUS)