erofs: update documentation

 - refine the filesystem overview for better description of recent
   new features like FSDAX and Fscache;

 - add the new `fsid' mount option;

 - fix some typos.

Link: https://lore.kernel.org/r/20220527070133.77962-1-hsiangkao@linux.alibaba.com


Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>