Clarify Gerrit behaviour when using customised gitweb configuration

When using a customised gitweb configuration, values must be supplied
for all of the `revision`, `branch`, `project` and `filehistory`
settings, otherwise the configuration will be disabled.

Add a note about this in the documentation.

Change-Id: I2f27b240e5f3fb5f3c11906bf580ff55423a8f00
diff --git a/Documentation/config-gitweb.txt b/Documentation/config-gitweb.txt
index 7ba15b8..711174f 100644
--- a/Documentation/config-gitweb.txt
+++ b/Documentation/config-gitweb.txt
@@ -245,6 +245,10 @@
 After updating `'$site_path'/etc/gerrit.config`, the Gerrit server must
 be restarted and clients must reload the host page to see the change.
 
+Note that when using a custom gitweb configuration, values must be
+specified for all of the `project`, `revision`, `branch` and `filehistory`
+settings, otherwise the configuration will not be used.
+
 Access Control
 ++++++++++++++