config.txt: document the semantics of hideRefs with namespaces

Right now, there is no clear definition of how transfer.hideRefs should
behave when a namespace is set. Explain that hideRefs prefixes match
stripped names in that case. This is how hideRefs patterns are currently
handled in receive-pack.

Signed-off-by: Lukas Fleischer <lfleischer@lfos.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

diff --git a/Documentation/config.txt b/Documentation/config.txt
index 391a0c3..993e3a9 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -2673,6 +2673,14 @@
 explicitly exposing it, even if an earlier entry marked it as hidden.
 If you have multiple hideRefs values, later entries override earlier ones
 (and entries in more-specific config files override less-specific ones).
++
+If a namespace is in use, the namespace prefix is stripped from each
+reference before it is matched against `transfer.hiderefs` patterns.
+For example, if `refs/heads/master` is specified in `transfer.hideRefs` and
+the current namespace is `foo`, then `refs/namespaces/foo/refs/heads/master`
+is omitted from the advertisements but `refs/heads/master` and
+`refs/namespaces/bar/refs/heads/master` are still advertised as so-called
+"have" lines.
 
 transfer.unpackLimit::
 	When `fetch.unpackLimit` or `receive.unpackLimit` are