git-tag(1): Refer to git-check-ref-format(1) for <name>

Explain briefly what characters are prohibited in tag <name>
and point to git-check-ref-format(1) manual page for
further information.

Signed-off-by: Jari Aalto <jari.aalto@cante.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-tag.txt

@@ -17,7 +17,10 @@ SYNOPSIS
 
 DESCRIPTION
 -----------
-Adds a 'tag' reference in `.git/refs/tags/`
+
+Adds a 'tag' reference in `.git/refs/tags/`.  The tag <name> must pass
+linkgit:git-check-ref-format[1] which basicly means that control characters,
+space, ~, ^, :, ?, *, [ and \ are prohibited.
 
 Unless `-f` is given, the tag must not yet exist in
 `.git/refs/tags/` directory.