Fix documentation of external PCM plugin SDK.
- Added protocl version check. The caller of snd_pcm_{io|ext}plug_create() must fill version field with SND_PCM_{IO|EXT}PLUG_VERSION beforehand. - Added poll_descriptors and poll_descriptors_count callbacks for ioplug.
Added more doxygen comments in pcm_extplug and pcm_ioplug codes.
Added the SDK for filter-type PCM plugins. Share some codes with ioplug.