drm/i915/guc: improve documentation

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 <[email protected]>
Cc: Michal Wajdeczko <[email protected]>
Cc: Matthew Brost <[email protected]>
Cc: Martin Peres <[email protected]>
Acked-by: Anna Karas <[email protected]>
Reviewed-by: Martin Peres <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]