Document enabling edit links by setting gerritUrl

Change-Id: I6ab7b304aae2dc1b1f9d6b81941d3a919351faf7

Documentation/config.md

diff --git a/Documentation/config.md b/Documentation/config.md
index 65727e5..6c5eac3 100644
--- a/Documentation/config.md
+++ b/Documentation/config.md
@@ -23,10 +23,15 @@
 
 `baseGitUrl` the base URL for git repositories.
 
-`gerritUrl`, URL prefix to linkify Gerrit `Change-Id` headers in commit
-messages. If you are using the Gerrit Gitiles plugin, this is set based on
-Gerrit's configuration.
-Default: `null`, do not link `Change-Id`.
+`gerritUrl`, URL prefix that when set is used to create:
+* Links from the `Change-Id` headers in commit messages to the corresponding
+  Gerrit change.
+* An edit link in file views that opens a new change in Gerrit in online edit
+  mode for that file. The link will only appear when you are browsing the file
+  at a branch, so the commitish must start with `refs/heads/`.
+
+> If you are using the Gerrit Gitiles plugin, this is set based on Gerrit's configuration.
+> Default: `null`, do not link `Change-Id` or show edit links.
 
 ```
 [gitiles]