Update and improve documentation of project.el commands

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.