Elaborate on why ':' is a bad idea in a ref name.

With the new cat-file syntax of 'v1.3.3:refs.c' we should mention
it as part of the reason why ':' is not permitted in a ref name.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-check-ref-format.txt

@@ -45,6 +45,8 @@ refname expressions (see gitlink:git-rev-parse[1]).  Namely:
 
 . colon `:` is used as in `srcref:dstref` to mean "use srcref\'s
   value and store it in dstref" in fetch and push operations.
+  It may also be used to select a specific object such as with
+  gitlink:git-cat-file[1] "git-cat-file blob v1.3.3:refs.c".
 
 
 GIT