drm/i915: Pimp the watermark documentation a bit

Document what each of the "raw" vs. "optimal" vs. "intermediate"
watermarks do.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201106173042.7534-4-ville.syrjala@linux.intel.com