Document gitweb URL set, cgi unset configuration

When using an external instance of gitweb (or gitiles), the gitweb.cgi
configuration variable has no reason to be set.  Setting it would cause
the internal GitwebServlet to also be provisioned, which would not be the
intended behavior in this configuration.

Change-Id: I597b0c290fbb091628798b31421beacaa7dde1e4
1 file changed