erofs: update documentation

 - update new features like bloom filter and DEFLATE.

 - add documentation for the long xattr name prefixes, which was
   landed upstream since v6.4.

Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20230928134852.31118-1-jefflexu@linux.alibaba.com


Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>