Update and improve documentation of project.el commands
* doc/emacs/custom.texi (Prefix Keymaps): Document 'project-prefix-map'. * doc/emacs/maintaining.texi (Project File Commands) (Switching Projects): Describe key bindings for the commands described in the sections. Document 'project-list-file'. (Project Buffer Commands): New section. * doc/emacs/emacs.texi (Top): Add Project sections to the detailed menu. * etc/NEWS: Add entries for project.el, and mark documented entries as appropriate.