I modified section 13.14 (The declare form) of the GNU Emacs Lisp
Reference Manual.  The manual didn't say how to make

    (declare (completion ...)
             (modes ...))

take effect; the original text is even somewhat misleading.  Simply
evaluating a (defun ...) form which includes the above declare form
won't make Emacs filter commands when asking for completion in M-x.

--
shynur