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>

Documentation/git-cvsserver.txt

@@ -31,6 +31,10 @@ over pserver for anonymous CVS access.
 
 CVS clients cannot tag, branch or perform GIT merges.
 
+git-cvsserver maps GIT branches to CVS modules. This is very different
+from what most CVS users would expect since in CVS modules usually represent
+one or more directories.
+
 INSTALLATION
 ------------