[PATCH] Documentation: sound: fix TAS675x doc build warnings

[Randy Dunlap]

Add tas675x.rst to the index file and extend the heading underline
to avoid build warnings:

Documentation/sound/codecs/tas675x.rst: WARNING: document isn't included in any toctree [toc.not_included]
Documentation/sound/codecs/tas675x.rst:659: WARNING: Title underline too short.
Overtemperature Shutdown (0x87)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ [docutils]

Fixes: ba46edca354e ("Documentation: sound: Add TAS675x codec mixer controls documentation")
Signed-off-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
---
Cc: Sen Wang <sen@xxxxxx>
Cc: Mark Brown <broonie@xxxxxxxxxx>
Cc: Liam Girdwood <lgirdwood@xxxxxxxxx>
Cc: Jaroslav Kysela <perex@xxxxxxxx>
Cc: Takashi Iwai <tiwai@xxxxxxxx>
Cc: linux-sound@xxxxxxxxxxxxxxx

 Documentation/sound/codecs/index.rst   |    1 +
 Documentation/sound/codecs/tas675x.rst |    2 +-
 2 files changed, 2 insertions(+), 1 deletion(-)

--- linux-next-20260429.orig/Documentation/sound/codecs/index.rst
+++ linux-next-20260429/Documentation/sound/codecs/index.rst
@@ -7,3 +7,4 @@ Codec-Specific Information
    :maxdepth: 2
 
    cs35l56
+   tas675x
--- linux-next-20260429.orig/Documentation/sound/codecs/tas675x.rst
+++ linux-next-20260429/Documentation/sound/codecs/tas675x.rst
@@ -656,7 +656,7 @@ These faults place affected channels int
 issues fault clear (register 0x01 bit 3) to allow recovery.
 
 Overtemperature Shutdown (0x87)
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
   ========  ==========================================
   Bits      Description