Revert "doc/sphinx: Enable keep_warnings"

This reverts commit 47d6d752b9e20dbe8a2acd22e887be81a6f39de9.

Commit f42ddca7bebc (doc-rst: kernel-doc directive, fix state machine
reporter) from Marcus Heiser provides a better fix, so this configuration
change is no longer needed.

Documentation/conf.py

@@ -125,7 +125,7 @@ pygments_style = 'sphinx'
 #modindex_common_prefix = []
 
 # If true, keep warnings as "system message" paragraphs in the built documents.
-keep_warnings = True
+#keep_warnings = False
 
 # If true, `todo` and `todoList` produce output, else they produce nothing.
 todo_include_todos = False