git-rebase: Clarify usage statement and copy it into the actual documentation.

git-rebase: Clarify usage statement and copy it into the actual documentation.

I found a paper thin man page for git-rebase, but was quite happy to
see something much more useful in the usage statement of the script
when I went there to find out how this thing worked. Here it is
cleaned up slightly and expanded a bit into the actual documentation.

Signed-off-by: Carl Worth <cworth@cworth.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>