ALSA: doc: usb-audio: Add doc for QUIRK_FLAG_SKIP_IFACE_SETUP

QUIRK_FLAG_SKIP_IFACE_SETUP was introduced into usb-audio before without
appropriate documentation, so add it.

Fixes: 38c3220 ("ALSA: usb-audio: Add QUIRK_FLAG_SKIP_IFACE_SETUP")
Cc: stable@vger.kernel.org
Signed-off-by: Rong Zhang <i@rong.moe>
Link: https://patch.msgid.link/20260302173300.322673-1-i@rong.moe
Signed-off-by: Takashi Iwai <tiwai@suse.de>

Author: Rongronggg9

Documentation/sound/alsa-configuration.rst

@@ -2372,6 +2372,10 @@ quirk_flags
           audible volume
         * bit 25: ``mixer_capture_min_mute``
           Similar to bit 24 but for capture streams
+        * bit 26: ``skip_iface_setup``
+          Skip the probe-time interface setup (usb_set_interface,
+          init_pitch, init_sample_rate); redundant with
+          snd_usb_endpoint_prepare() at stream-open time
 
 This module supports multiple devices, autoprobe and hotplugging.