Add documentation for the markdown.allowiframe setting

Change-Id: I50263027f42d00c4316b0aa003d00e53ac44d0c3

Documentation/markdown.md

diff --git a/Documentation/markdown.md b/Documentation/markdown.md
index fdd34a9..baf728f 100644
--- a/Documentation/markdown.md
+++ b/Documentation/markdown.md
@@ -619,7 +619,8 @@
 _(optional)_
 
 Embedded source URLs must also be whitelisted by the Gitiles
-`markdown.allowiframe` configuration variable.
+[`markdown.allowiframe`](/Documentation/config.md#IFrames) configuration
+variable.
 
 ## Site layout