bookmarks: update help text since moving into core
Clarifies the help text for the bookmarks command regarding the
requirements for pushing or pulling bookmarks.
$ hg init dir
$ cd dir
$ echo bleh > bar
$ hg add bar
$ hg ci -m 'add bar'
$ hg cp bar foo
$ echo >> bar
$ hg ci -m 'cp bar foo; change bar'
$ hg debugrename foo
foo renamed from bar:26d3ca0dfd18e44d796b564e38dd173c9668d3a9
$ hg debugindex .hg/store/data/bar.i
rev offset length base linkrev nodeid p1 p2
0 0 6 0 0 26d3ca0dfd18 000000000000 000000000000
1 6 7 1 1 d267bddd54f7 26d3ca0dfd18 000000000000