ALSA: hda - document the ELD proc interface

Describe what ELD proc interface provides and how to fix incorrect values.
Signed-off-by: Wu Fengguang <wfg@linux.intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

Documentation/sound/alsa/Procfile.txt

@@ -153,6 +153,16 @@ card*/codec#*
 	Shows the general codec information and the attribute of each
 	widget node.
 
+card*/eld#*
+	Available for HDMI or DisplayPort interfaces.
+	Shows ELD(EDID Like Data) info retrieved from the attached HDMI sink,
+	and describes its audio capabilities and configurations.
+
+	Some ELD fields may be modified by doing `echo name hex_value > eld#*`.
+	Only do this if you are sure the HDMI sink provided value is wrong.
+	And if that makes your HDMI audio work, please report to us so that we
+	can fix it in future kernel releases.
+
 
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