Revise QTest::qWait() docs to encourage a preference for QTRY_*() macros

There's a fairly widespread view that qWait() is usually a source of eventual pain, rather than the right tool for the job; while it may be a bit premature to actively deprecate it, we could at least have its docs mention the QTRY_*() macros, and maybe even actively encourage the reader to prefer them, where applicable.