Document that QAudioOutput can cause a crash when working with multiple threads in Symbian

Symbian CMMFDevSound assumes that each instance is created and used from one thread only.
If a DevSound instance is created in one thread, the first call coming from another thread will cause a KERN-EXEC 0 panic.

This limitation/particularity of DevSound usage should be resolved (probably by wrapping devsound instance inside own thread), avoided (by denying DevSound calls coming from "other" threads) or at least "advertised" (by mentioning this limitation in documentation as a Symbian specific particularity).