drm/i915: expand on the kernel-doc for cache_dirty

Add some details around non-LLC platforms and cflushing, when dealing
with the flush-on-acquire, which is potentially security sensitive.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211018174508.2137279-7-matthew.auld@intel.com