some doc updates

some doc updates

1) talk about "git merge" instead of "git pull ."

2) suggest "git repo-config" instead of directly editing config files

3) echo "URL: blah" > .git/remotes/foo is obsolete and should be
   "git repo-config remote.foo.url blah"

4) support for partial URL prefix has been removed (see commit
   ea560e6d64374ec1f6c163c276319a3da21a1345) so drop mention of it.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>