doc: Comment out language option

... because explicit language=None causes the below warning since
Sphinx 5.0.0.

Invalid configuration value found: 'language = None'. Update your
configuration to a valid language code. Falling back to 'en' (English).

Change-Id: Ieca6a2cb9724346ed4ee40a20335a02d737a3cd4

doc/source/conf.py

@@ -74,7 +74,7 @@
 #
 # This is also used if you do content translation via gettext catalogs.
 # Usually you set "language" from the command line for these cases.
-language = None
+# language = None
 
 # List of patterns, relative to source directory, that match files and
 # directories to ignore when looking for source files.