docs: sphinx-extensions: add metadata parallel-safe

The setup() function of a Sphinx-extension can return a dictionary. This
is treated by Sphinx as metadata of the extension [1].

With metadata "parallel_read_safe = True" a extension is marked as
save for "parallel reading of source". This is needed if you want
build in parallel with N processes. E.g.:

  make SPHINXOPTS=-j4 htmldocs

will no longer log warnings like:

  WARNING: the foobar extension does not declare if it is safe for
  parallel reading, assuming it isn't - please ask the extension author
  to check and make it explicit.

Add metadata to extensions:

* kernel-doc
* flat-table
* kernel-include

[1] http://www.sphinx-doc.org/en/stable/extdev/#extension-metadata

Signed-off-by: Markus Heiser <markus.heiser@darmarIT.de>
Tested-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/sphinx/rstFlatTable.py

@@ -73,6 +73,12 @@ def setup(app):
     roles.register_local_role('cspan', c_span)
     roles.register_local_role('rspan', r_span)
 
+    return dict(
+        version = __version__,
+        parallel_read_safe = True,
+        parallel_write_safe = True
+    )
+
 # ==============================================================================
 def c_span(name, rawtext, text, lineno, inliner, options=None, content=None):
 # ==============================================================================