Add new 'declare' forms for command completion predicates

Add new 'declare' forms for command completion predicates

* doc/lispref/functions.texi (Declare Form): Document the new
`completion' and `modes' declarations.
* lisp/simple.el (completion-with-modes-p): New helper functions.

* lisp/emacs-lisp/byte-run.el (byte-run--set-completion)
(byte-run--set-modes):
(defun-declarations-alist): New declarations for `completion' and
`modes'.