drm/i915/guc: Improve the GuC documentation & comments about proxy submissions

While at it, fix a typo (s/ring_lcra/ring_lrca) and improve the naming of one
firware interface field (s/ring_tail/submit_element_info, since it can contain
more than just the ring tail).

No change in functionality.

v2:
  - Remove reference to "unique user" of the GuC (Daniele)
  - Keep mention to renaming from "GuC context" to "client" (Daniele)

Signed-off-by: Oscar Mateo <oscar.mateo@intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>