qdoc: Document how relative paths given to \snippet are resolved

It seems that currently, relative path for the \snippet command are only resolved against exampledirs in the .qdocconf file. This is somewhat surprising, as we nowadays recommend that snippets & examples are kept separately.

Suggestions:

• Document how \snippet is resolved, e.g. in https://doc.qt.io/qt-6/07-0-qdoc-commands-includingexternalcode.html#snippet or at the top-level page.
• Consider introducing a separate 'snippetdirs' setting in .qdocconf