Improve documentation of project.el commands

Improve documentation of project.el commands

* lisp/progmodes/project.el (project-find-regexp): Require 'grep'
to be able to call 'grep-read-files'.
(project-search, project-query-replace-regexp): Doc fixes.

* doc/emacs/maintaining.texi (Projects): New section.
* doc/emacs/emacs.texi (Top): Add "Projects" to the detailed menu.