drm/i915/guc: improve documentation

Add a short description of what we expect from GuC and some minor
improvements to existing documentation. Also remove a comment about a
difference between GuC and HuC that is not true anymore.

v2: add that the GuC is not mandatory (Martin)
v3: add extra newline for better text organization (Martin)

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Martin Peres <martin.peres@linux.intel.com>
Acked-by: Anna Karas <anna.karas@intel.com>
Reviewed-by: Martin Peres <martin.peres@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191014183602.3643-2-daniele.ceraolospurio@intel.com