ALSA: docs: Extend module parameters description

Common ALSA module parameters look a little bit confusing because of the
description lacking, and it took me a while to understand the purpose of
their existence. To figure it out I asked the question about them to the
"alsa-devel" mailing list, and Takashi Iwai answered me with the text I
appended to the ALSA documentation in this patch.

These common module parameters aren't used a lot nowadays, but as I
understand they are important for providing compatibility with some
existing user-space apps. So in my opinion it is a good idea to document
why we need them.
Signed-off-by: Ivan Orlov <ivan.orlov0322@gmail.com>
Link: https://lore.kernel.org/r/20230501101634.476297-1-ivan.orlov0322@gmail.comSigned-off-by: Takashi Iwai <tiwai@suse.de>

Documentation/sound/alsa-configuration.rst

@@ -133,6 +133,19 @@ enable
     enable card;
     Default: enabled, for PCI and ISA PnP cards
 
+These options are used for either specifying the order of instances or
+controlling enabling and disabling of each one of the devices if there
+are multiple devices bound with the same driver. For example, there are
+many machines which have two HD-audio controllers (one for HDMI/DP
+audio and another for onboard analog). In most cases, the second one is
+in primary usage, and people would like to assign it as the first
+appearing card. They can do it by specifying "index=1,0" module
+parameter, which will swap the assignment slots.
+
+Today, with the sound backend like PulseAudio and PipeWire which
+supports dynamic configuration, it's of little use, but that was a
+help for static configuration in the past.
+
 Module snd-adlib
 ----------------