drm/doc: link style-guide to doc-guide

Our style guide should only be the delta compared to the overall one.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1483008275-29383-1-git-send-email-daniel.vetter@ffwll.ch

Documentation/gpu/introduction.rst

@@ -49,3 +49,5 @@ section name should be all upper-case or not, and whether it should end
 in a colon or not. Go with the file-local style. Other common section
 names are "Notes" with information for dangerous or tricky corner cases,
 and "FIXME" where the interface could be cleaned up.
+
+Also read the :ref:`guidelines for the kernel documentation at large <doc_guide>`.