shell: Document that 'cvs server' is a valid command

git-shell's man page explicitly lists all allowed commands, but 'cvs
server' was missing. Add it.

Signed-off-by: Lars Noschinski <lars@public.noschinski.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-shell.txt

@@ -18,8 +18,9 @@ of server-side GIT commands implementing the pull/push functionality.
 The commands can be executed only by the '-c' option; the shell is not
 interactive.
 
-Currently, only the 'git-receive-pack' and 'git-upload-pack' commands
-are permitted to be called, with a single required argument.
+Currently, only three commands are permitted to be called, 'git-receive-pack'
+'git-upload-pack' with a single required argument or 'cvs server' (to invoke
+'git-cvsserver').
 
 Author
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