mediawiki.Title: Correct documentation structure for Title.exist

Apparently the way we were using the documentation, with sub-properties
of properties, only worked due to the way that JSDuck v4.x happened to
be implemented, and was unsupported; as of JSDuck v5.x this no longer
works, so instead, change the form used so as to document the property
as a single item, as suggested in response to our complaint:

  https://github.com/senchalabs/jsduck/issues/551

Change-Id: If0c66e6e8a904769ee857c1ee94377dc13000d41