Documentation: clarify / requirement in 'git check-ref-format'

'git check-ref-format' checks for the presence of at least one '/', the
idea being that there should be no refs directly below 'refs/', so there
should be a category like 'heads/' or 'tags/' in a refname.

Try and make this clearer in the man page.
Signed-off-by: NMichael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-check-ref-format.txt

@@ -25,6 +25,10 @@ imposes the following rules on how references are named:
   grouping, but no slash-separated component can begin with a
   dot `.`.
 
+. They must contain at least one `/`. This enforces the presence of a
+  category like `heads/`, `tags/` etc. but the actual names are not
+  restricted.
+
 . They cannot have two consecutive dots `..` anywhere.
 
 . They cannot have ASCII control characters (i.e. bytes whose