ALSA: doc: Add the description of quirk_flags option for snd-usb-audio

Just briefly described about the new option.

Link: https://lore.kernel.org/r/20210729074404.19728-6-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

Documentation/sound/alsa-configuration.rst

@@ -2252,6 +2252,25 @@ delayed_register
     The driver prints a message like "Found post-registration device
     assignment: 1234abcd:04" for such a device, so that user can
     notice the need.
+quirk_flags
+    Contains the bit flags for various device specific workarounds.
+    Applied to the corresponding card index.
+        * bit 0: Skip reading sample rate for devices
+        * bit 1: Create Media Controller API entries
+        * bit 2: Allow alignment on audio sub-slot at transfer
+        * bit 3: Add length specifier to transfers
+        * bit 4: Start playback stream at first in implement feedback mode
+        * bit 5: Skip clock selector setup
+        * bit 6: Ignore errors from clock source search
+        * bit 7: Indicates ITF-USB DSD based DACs
+        * bit 8: Add a delay of 20ms at each control message handling
+        * bit 9: Add a delay of 1-2ms at each control message handling
+        * bit 10: Add a delay of 5-6ms at each control message handling
+        * bit 11: Add a delay of 50ms at each interface setup
+        * bit 12: Perform sample rate validations at probe
+        * bit 13: Disable runtime PM autosuspend
+        * bit 14: Ignore errors for mixer access
+        * bit 15: Support generic DSD raw U32_BE format
 
 This module supports multiple devices, autoprobe and hotplugging.
 
@@ -2261,11 +2280,14 @@ check.
 
 NB: ``ignore_ctl_error=1`` may help when you get an error at accessing
 the mixer element such as URB error -22.  This happens on some
-buggy USB device or the controller.
+buggy USB device or the controller.  This workaround corresponds to
+the ``quirk_flags`` bit 14, too.
 
-NB: quirk_alias option is provided only for testing / development.
+NB: ``quirk_alias`` option is provided only for testing / development.
 If you want to have a proper support, contact to upstream for
 adding the matching quirk in the driver code statically.
+Ditto for ``quirk_flags``.  If a device is known to require specific
+workarounds, please report to the upstream.
 
 Module snd-usb-caiaq
 --------------------