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

Documentation/filesystems/erofs.rst

@@ -120,6 +120,8 @@ dax={always,never}     Use direct access (no page cache).  See
 dax                    A legacy option which is an alias for ``dax=always``.
 device=%s              Specify a path to an extra device to be used together.
 fsid=%s                Specify a filesystem image ID for Fscache back-end.
+domain_id=%s           Specify a domain ID in fscache mode so that different images
+                       with the same blobs under a given domain ID can share storage.
 ===================    =========================================================
 
 Sysfs Entries