drm/i915/uapi: convert i915_user_extension to kernel doc

drm/i915/uapi: convert i915_user_extension to kernel doc

Add some example usage for the extension chaining also, which is quite
nifty.

v2: (Daniel)
  - clarify that the name is just some integer, also document that the
    name space is not global
v3: prefer kernel-doc references for structs

Suggested-by: Daniel Vetter <[email protected]>
Signed-off-by: Matthew Auld <[email protected]>
Cc: Joonas Lahtinen <[email protected]>
Cc: Jordan Justen <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Kenneth Graunke <[email protected]>
Cc: Jason Ekstrand <[email protected]>
Cc: Dave Airlie <[email protected]>
Cc: [email protected]
Cc: [email protected]
Reviewed-by: Daniel Vetter <[email protected]>
Reviewed-by: Jason Ekstrand <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]