cvsserver: Document the GIT branches -> CVS modules mapping more prominently

cvsserver: Document the GIT branches -> CVS modules mapping more prominently

Add a note about the branches -> modules mapping to LIMITATIONS because
I really think it should be noted there and not just at the end of
the installation step-by-step HOWTO.

I used "git branches" there and changed "heads" to "branches" in
my section about database configuration. I'm reluctant to replace
all occourences of "head" with "branch" though because you always
have to say "git branch" because CVS also has the concept of
branches. You can say "head" though, because there is no such
concept in CVS. In all the existing occourences of head other than
the one I changed I think "head" flows better in the text.

Signed-off-by: Frank Lichtenheld <frank@lichtenheld.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>