docs/config-themes: clarify and link to JavaScript plugin dev

I found it very difficult to figure out why etc/GerritSite.css wasn't
making any style modifications on non-login pages. I overlooked the fact
that this section only applies to login screens. Clarifying that section
and linking to the JavaScript Plugin Development and API document from
this Themes document would help a lot.

Change-Id: Ibd2a481527a4a9dbc44075c4bd2c54432a6ccbe6
diff --git a/Documentation/config-themes.txt b/Documentation/config-themes.txt
index a83c747..73fb1bc 100644
--- a/Documentation/config-themes.txt
+++ b/Documentation/config-themes.txt
@@ -4,11 +4,12 @@
 the browser, allowing organizations to alter the look and
 feel of the application to fit with their general scheme.
 
-== HTML Header/Footer and CSS
+== HTML Header/Footer and CSS for login screens
 
-The HTML header, footer and CSS may be customized for login
-screens (LDAP, OAuth, OpenId) and the internally managed
-Gitweb servlet.
+The HTML header, footer, and CSS may be customized for login screens (LDAP,
+OAuth, OpenId) and the internally managed Gitweb servlet. See
+link:pg-plugin-dev.txt[JavaScript Plugin Development and API] for documentation
+on modifying styles for the rest of Gerrit (not login screens).
 
 At startup Gerrit reads the following files (if they exist) and
 uses them to customize the HTML page it sends to clients: