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
1 file changed