doc: Clarify documentation for ParserOptions::getUserLangObj

It no longer produces inconsistent link tables, that was fixed
in r89706 by setting the parser to the default language when
saving, and then when viewing it might reparse for a different
uselang if needed (without saving link updates).

Of course, that isn't pretty and it should still be discouraged.

Change-Id: I599a7162fed458713ffa3a754086c6a2ffdebe07