erofs: update documentation

- Refine highlights for main features;

- Add multi-reference pclusters and fragment description.

Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Yue Hu <huyue2@coolpad.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/20221130095605.4656-1-hsiangkao@linux.alibaba.com