ALSA: docs: A few more words for PCM XRUN handling and stream locks

Enhance the documents about the PCM, missing descriptions for a couple
of helpers like snd_pcm_period_elapsed_under_stream_lock() and
snd_pcm_stop_xrun().

Link: https://lore.kernel.org/r/20230323065237.5062-4-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

Documentation/sound/kernel-api/writing-an-alsa-driver.rst

@@ -2215,6 +2215,12 @@ Typical code would be like:
               return IRQ_HANDLED;
       }
 
+Also, when the device can detect a buffer underrun/overrun, the driver
+can notify the XRUN status to the PCM core by calling
+:c:func:`snd_pcm_stop_xrun()`. This function stops the stream and sets
+the PCM state to ``SNDRV_PCM_STATE_XRUN``. Note that it must be called
+outside the PCM stream lock, hence it can't be called from the atomic
+callback.
 
 
 High frequency timer interrupts
@@ -2294,8 +2300,9 @@ mutexes or semaphores instead.
 As already seen, some pcm callbacks are atomic and some are not. For
 example, the ``hw_params`` callback is non-atomic, while ``trigger``
 callback is atomic. This means, the latter is called already in a
-spinlock held by the PCM middle layer. Please take this atomicity into
-account when you choose a locking scheme in the callbacks.
+spinlock held by the PCM middle layer, the PCM stream lock. Please
+take this atomicity into account when you choose a locking scheme in
+the callbacks.
 
 In the atomic callbacks, you cannot use functions which may call
 :c:func:`schedule()` or go to :c:func:`sleep()`. Semaphores and
@@ -2318,6 +2325,13 @@ in the PCM core instead of spin and rwlocks, so that you can call all PCM
 functions safely in a non-atomic
 context.
 
+Also, in some cases, you might need to call
+:c:func:`snd_pcm_period_elapsed()` in the atomic context (e.g. the
+period gets elapsed during ``ack`` or other callback). There is a
+variant that can be called inside the PCM stream lock
+:c:func:`snd_pcm_period_elapsed_under_stream_lock()` for that purpose,
+too.
+
 Constraints
 -----------