Fix docs for getOffsetWidth/Height.

Repost of Rietveld issue 1586803.
Patch by: stephenh

Review at http://gwt-code-reviews.appspot.com/1611803

Review by: rchandia@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10796 8db76d5a-ed1c-0410-87a9-c151d255dfc7