erofs: add documentation for 'domain_id' mount option

Since the EROFS share domain feature for fscache mode has been available
since Linux v6.1, let's add documentation for 'domain_id' mount option.

Cc: linux-doc@vger.kernel.org
Reviewed-by: Jia Zhu <zhujia.zj@bytedance.com>
Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/20230112065431.124926-2-jefflexu@linux.alibaba.com
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>