A regenerated git-cvsserver database is at risk of having different
CVS revision numbers from an incrementally updated database.  Mention
this in the the documentation, and remove an erroneous statement
to the contrary.
Signed-off-by: NMatthew Ogilvie <mmogilvi_git@miniinfo.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>