FROMGIT: erofs: update documentation about data compression

Add more description about (NON)HEAD lclusters, and the new big
pcluster feature.

Link: https://lore.kernel.org/r/20210511084414.21305-1-xiang@kernel.org


Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <xiang@kernel.org>
(cherry picked from commit 85278268
 https://android.googlesource.com/kernel/common

 android13-5.10)

BUG=b:273658215
TEST=`tast run $DUT arc.Boot.vm`
TEST=`tast run $DUT arc.Playstore.vm`

Change-Id: I97e85a045f6dcdb86d829aef646ac96d58f5fd0a
Signed-off-by: Ryuichiro Chiba <chibar@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/4583877


Reviewed-by: Sean Paul <sean@poorly.run>
Reviewed-by: Hikaru Nishida <hikalium@chromium.org>