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.