thread-loop: clarify some docs about locking

src/pipewire/thread-loop.h

@@ -61,6 +61,11 @@ extern "C" {
  *
  * All events and callbacks are called with the thread lock held.
  *
+ * An exception to this is for the data processing callbacks, which are
+ * explcitly marked as being emitted from the data realtime threads. One
+ * such callback is the \ref pw_stream::process() callback when the
+ * \ref PW_STREAM_FLAG_RT_PROCESS is set.
+ *
  */
 /** \defgroup pw_thread_loop Thread Loop
  *