Improve documentation of Dired's 'A' and 'Q' commands

Improve documentation of Dired's 'A' and 'Q' commands

* lisp/dired-aux.el (dired-do-find-regexp)
(dired-do-find-regexp-and-replace): Mention
'grep-find-ignored-files' and 'vc-directory-exclusion-list', and
also the fact that REGEXP should be palatable by Grep.  (Bug#23426)
* lisp/dired.el: Update the corresponding autoload forms.

* doc/emacs/dired.texi (Operating on Files): Mention
'grep-find-ignored-files' and 'vc-directory-exclusion-list'.
(Bug#23429)