Clarify documentation of git-cvsserver, particularly in relation to git-shell

Clarify documentation of git-cvsserver, particularly in relation to git-shell

For SSH clients restricted to git-shell, CVS_SERVER does not have to be
specified, because git-shell understands the default value of 'cvs' to
mean git-cvsserver'. This makes it totally transparent to CVS users, but
the instruction to set up CVS access for people with real shell access
does not apply.

Previous wording mentioning GIT_AUTHOR, GIT_COMMITTER variables was
unclear that we really meant GIT_AUTHOR_(NAME|EMAIL), etc.

Note that the .ssh/environment file is a good place to set these, and that
the .bashrc is shell-specific. Add a bit of text to differentiate cvs -d
(setting CVSROOT) from cvs co -d (setting the name of the newly checked
out directory).  Removed an extra 'Example:' string.

Signed-off-by: Scott Collins <scc@ScottCollins.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>