Add some info to gitweb access control docs.

In order for a user to be allowed to view any gitweb information about a
repository, they must have read access to all references. Now this fact
is documented.

Change-Id: I8bb738678af89599318bbe114e35b2a63fc09f6f

Documentation/config-gitweb.txt

diff --git a/Documentation/config-gitweb.txt b/Documentation/config-gitweb.txt
index 7ba15b8..cb5e1e3 100644
--- a/Documentation/config-gitweb.txt
+++ b/Documentation/config-gitweb.txt
@@ -62,6 +62,12 @@
 Access controls for internally managed gitweb page views are enforced
 using the standard project READ +1 permission.
 
+Also, in order for a user to be able to view any gitweb information for a
+project, the user must be able to read all referencess (including
+refs/meta/config, refs/meta/dashboards/*, etc.). If you have exclusive read
+permissions for any references, make sure to include all parties that should be
+able to read the gitweb info for any of the branches in that project.
+
 External/Unmanaged gitweb
 ~~~~~~~~~~~~~~~~~~~~~~~~~