docs: Set the Sphinx default highlight language to "guess"

This should eliminate a whole class of markup warnings, at the cost of
occasionally amusing markup choices; we'll have to see if it works out.

Suggested-by: Markus Heiser <markus.heiser@darmarIT.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/conf.py

@@ -131,7 +131,7 @@ pygments_style = 'sphinx'
 todo_include_todos = False
 
 primary_domain = 'C'
-highlight_language = 'C'
+highlight_language = 'guess'
 
 # -- Options for HTML output ----------------------------------------------