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>