drm/i915/huc: improve documentation

Better explain the usage of the microcontroller and what i915 is
responsible of. While at it, fix the documentation for the auth
function, which doesn't do any pinning anymore.

v2: add a comment on HuC being optional and descrive how HuC accesses
    memory (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: 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/20191014183602.3643-3-daniele.ceraolospurio@intel.com