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