cvsserver: Note that CVS_SERVER can also be specified as method variable

cvsserver: Note that CVS_SERVER can also be specified as method variable

Reasonably new versions of the cvs CLI client allow one to
specifiy CVS_SERVER as a method variable directly in
CVSROOT. This is way more convinient than using an
environment variable since it gets saved in CVS/Root.

Since I only discovered this by accident I guess there
might be others out there that learnt CVS on the 1.11
series (or even earlier) and profit from such a note
about cvs improvements in the last couple years.

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