drm/i915/guc: Add GuC kernel doc

Add GuC kernel doc for all structures added thus far for GuC submission
and update the main GuC submission section with the new interface
details.

v2:
 - Drop guc_active.lock DOC
v3:
 - Fixup a few kernel doc comments (Daniele)
v4 (Daniele):
 - Implement doc suggestions from John
 - Add kerneldoc for all members of the GuC structure and pull the file
   in i915.rst
v5 (Daniele):
 - Implement new doc suggestions from John

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210909164744.31249-24-matthew.brost@intel.com