control: doc - create controls plugin page

Signed-off-by: Jaroslav Kysela <perex@perex.cz>

include/control_plugin.h

@@ -0,0 +1,49 @@
+/**
+ * \file include/control_plugin.h
+ * \brief Common control plugin code
+ * \author Jaroslav Kysela <perex@perex.cz>
+ * \date 2021
+ *
+ * Application interface library for the ALSA driver.
+ * See the \ref control_plugins page for more details.
+ *
+ * \warning Using of contents of this header file might be dangerous
+ *	    in the sense of compatibility reasons. The contents might be
+ *	    freely changed in future.
+ */
+/*
+ *   This library is free software; you can redistribute it and/or modify
+ *   it under the terms of the GNU Lesser General Public License as
+ *   published by the Free Software Foundation; either version 2.1 of
+ *   the License, or (at your option) any later version.
+ *
+ *   This program is distributed in the hope that it will be useful,
+ *   but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *   GNU Lesser General Public License for more details.
+ *
+ *   You should have received a copy of the GNU Lesser General Public
+ *   License along with this library; if not, write to the Free Software
+ *   Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ *
+ */
+
+#ifndef __ALSA_CONTROL_PLUGIN_H
+#define __ALSA_CONTROL_PLUGIN_H
+
+/**
+ * \defgroup Control_Plugins Primitive Control Plugins
+ * \ingroup Control
+ * See the \ref control_plugins page for more details.
+ * \{
+ */
+
+/*
+ * Control HW
+ */
+int snd_ctl_hw_open(snd_ctl_t **handle, const char *name, int card, int mode);
+int _snd_ctl_hw_open(snd_ctl_t **handlep, char *name, snd_config_t *root ATTRIBUTE_UNUSED, snd_config_t *conf, int mode);
+
+/** \} */
+
+#endif /* __ALSA_CONTROL_PLUGIN_H */