Removes mention of isMandatory in list of d2wKeys.

This component does not honour the isMandatory key, as discussed on
the mailing list in 2010, 2013.

http://permalink.gmane.org/gmane.comp.web.webobjects.wonder-disc/15336
Merge pull request #424 from paulhoadley/d2w-javadoc-fix

Pulls 'isMandatory' key (which is no longer honoured) from the Javadocs.