decodebin2: Update documentation of the autoplug-* signals

decodebin2: Update documentation of the autoplug-* signals

Add notes about the behaviour if multiple signal handlers are connected.
For most autoplug-* signals only the first signal handler will ever
be invoked.

Also add to the autoplug-sort docs that the signal handler can return NULL
to specify that the order should change and other handlers get the chance
to sort the array.