Use grep-find-ignored-directories instead of vc-directory-exclusion-list

Use grep-find-ignored-directories instead of vc-directory-exclusion-list

* lisp/dired-aux.el (dired-do-find-regexp):
Use grep-find-ignored-directories instead of
vc-directory-exclusion-list.  The result should be functionally
similar (the former uses the latter as the default value), but it
should be more consistent and appropriate WRT user
customizations.
(dired-do-find-regexp-and-replace): Update the docstring.

* lisp/dired.el: Update the corresponding autoloads.

* doc/emacs/dired.texi (Operating on Files): Update the
documentation accordingly.