Add documentation for mod_suexec. And add a note in the old docs

about User that SuexecUserGroup now replaces putting User/Group directives
inside of VirtualHosts.
PR:	7634


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@89499 13f79535-47bb-0310-9956-ffa450edef68