output/alsa: add option "close_on_pause"

This allows keeping the ALSA PCM open even if playback is paused.  As
a side effect, this allows using the "always_on" option with ALSA
outputs, because "always_on" pauses the output.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1623

doc/plugins.rst

@@ -896,6 +896,11 @@ The `Advanced Linux Sound Architecture (ALSA) <http://www.alsa-project.org/>`_ p
        
        Example: "96000:16:* 192000:24:* dsd64:*=dop *:dsd:*".
 
+   * - **close_on_pause yes|no**
+     - Close the ALSA device while playback is paused?  This defaults
+       to *yes* because this allows other applications to use the
+       device while MPD is paused.
+
 The according hardware mixer plugin understands the following settings:
 
 .. list-table::

doc/user.rst

@@ -532,7 +532,8 @@ The following table lists the audio_output options valid for all plugins:
      - If set to yes, then :program:`MPD` attempts to keep this audio
        output always open.  Instead of closing at the end
        of playback, it puts the device in "pause" mode.  This works
-       only with output plugins that suport "pause" mode.
+       only with output plugins that suport "pause" mode (see
+       :ref:`ALSA option "close_on_pause" <alsa_plugin>`).
        This may be useful for streaming servers, when you don't want
        to disconnect all listeners even when playback is accidentally
        stopped.