Look for gitweb in /usr/share/gitweb

Debian based systems seem to install the gitweb CSS and logo
into /usr/share/gitweb.  So look for these files before the
older /var/www assumption when using the system supplied CGI.

Bug: issue 494
Change-Id: I03d470b3be095aaaa98ebf7ef66d5b29e712eb73
Signed-off-by: Shawn O. Pearce <sop@google.com>

Documentation/config-gitweb.txt

diff --git a/Documentation/config-gitweb.txt b/Documentation/config-gitweb.txt
index f72efaf..eb43733 100644
--- a/Documentation/config-gitweb.txt
+++ b/Documentation/config-gitweb.txt
@@ -34,6 +34,17 @@
 `'$site_path'/etc/gitweb_config.perl`, as this file is loaded as
 part of the generated configuration file.
 
+Logo and CSS
+~~~~~~~~~~~~
+
+If the package-manager installed CGI (`/usr/lib/cgi-bin/gitweb.cgi`)
+is being used, the stock CSS and logo files will be served from
+either `/usr/share/gitweb` or `/var/www`.
+
+Otherwise, Gerrit expects `gitweb.css` and `git-logo.png` to be found
+in the same directory as the CGI script itself.  This matches with
+the default source code distribution, and most custom installations.
+
 Access Control
 ~~~~~~~~~~~~~~