qdoc: Add warnings for linking / formatting commands used within section titles

Currently QDoc allow linking and formatting commands to be used within section titles (the argument for \section1..N commands).

This practice is not encouraged however, as it's often visually distracting and the same information would work better in the section content.

The only situation where it's useful and justifiable is API reference-style documentation where argument/return types and/or values of a signature are styled/linked to, similar to how QDoc formats the C++/QML reference it generates. This is an exception and rarely used, so we should add warnings whenever such commands are used within section titles.