Clarify that ParserOutput::getDisplayTitle() returns HTML.

Clarify that ParserOutput::getDisplayTitle() returns HTML.

This change only improves documentation, it does not touch code.

For reference:
* CoreParserFunctions::displaytitle generates HTML, with &, < and so on
property escaped. OutputPage::setPageTitle() treats the input as wikitext,
which works for both plain text and HTML.

* Wikibase calls ParserOutput::setDisplayTitle() with HTML including <span>
structures.

In the API action=query&prop=pageprops returns HTML. There doesn't seem to
be a good place to document that fact, though.

Change-Id: I043b26b82f066abe2830a81d3bc073543b3748d8