drm/i915: Add link to documentation in i915_gem_execbuffer.c

Add the documentation of "DOC: User command execution" of
i915_gem_execbuffer.c into a new section in i915.rst.
Signed-off-by: Kevin Rogovin <kevin.rogovin@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
[Joonas: correcting the patch title]
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1523001957-6427-3-git-send-email-kevin.rogovin@intel.com

Documentation/gpu/i915.rst

@@ -364,6 +364,12 @@ Batchbuffer Pools
 .. kernel-doc:: drivers/gpu/drm/i915/i915_gem_batch_pool.c
    :internal:
 
+User Batchbuffer Execution
+--------------------------
+
+.. kernel-doc:: drivers/gpu/drm/i915/i915_gem_execbuffer.c
+   :doc: User command execution
+
 Logical Rings, Logical Ring Contexts and Execlists
 --------------------------------------------------