alsa-mixer: Improve documentation in texas-instruments-pcm2902.conf

Part-of: <https://gitlab.freedesktop.org/pulseaudio/pulseaudio/-/merge_requests/667>

src/modules/alsa/mixer/profile-sets/texas-instruments-pcm2902.conf

@@ -13,22 +13,26 @@
 # You should have received a copy of the GNU Lesser General Public License
 # along with PulseAudio; if not, see <http://www.gnu.org/licenses/>.
 
-; Behringer U-Phoria UMC22
+; Texas Instruments PCM2902
 ;
-; Default mapping only allows to use stereo input and sound card has two
-; physical input channels.
+; This is a generic chip used in multiple products, including at least
+; Behringer U-Phoria UMC22, Intopic Jazz-UB700 and some unbranded "usb mini
+; microphone".
 ;
-; However in case of only using a single input channel (like condenser
-; microphone) only one channel will have any sound, which is often
-; inconvenient for casual use.
+; Behringer UMC22 has stereo input (representing two physical mono inputs),
+; others have mono input.
 ;
-; This config includes mono input options which makes it much more
-; friendly in single input configuration.
+; Since the UMC22 card has only stereo input PCM device but is commonly used
+; with mono mics, we define special mono mappings using "mono,aux1" and
+; "aux1,mono" channel maps. If we had only had the standard stereo input
+; mapping, the user would have to record stereo tracks with one channel silent,
+; which would be inconvenient.
 ;
 ; This config also removes default digital input/output mappings that do
-; not physically exist on this card.
+; not physically exist on cards that we've seen so far.
 ;
-; Added by Nazar Mokrynskyi <nazar@mokrynskyi.com>
+; Originally added by Nazar Mokrynskyi <nazar@mokrynskyi.com> for Behringer
+; UMC22.
 
 [General]
 auto-profiles = yes