Fixes for the inline documentation editor

Fixes most of the issues noted in comments to I5c3bc39a.

* overflow: auto on the <textarea>
* Heading replaced with placeholder
* The save button becomes active after editing
* <textarea> is automatically focused when opening the editor

Change-Id: I1978af2f4f5c0485dbc1292f3f93fa00d017ed43
Merge "Fixes for the inline documentation editor"