erofs: add missing documentation about directio mount option

Document the `directio` mount option for file-backed mounts, because
recent users need this and this mount option has been available since
commit 6422cde ("erofs: use buffered I/O for file-backed mounts
by default") without proper documentation.

Reported-by: Yuxuan Liu <cdjddzy@foxmail.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

Author: hsiangkao

Documentation/filesystems/erofs.rst

@@ -125,6 +125,8 @@ dax={always,never}     Use direct access (no page cache).  See
                        Documentation/filesystems/dax.rst.
 dax                    A legacy option which is an alias for ``dax=always``.
 device=%s              Specify a path to an extra device to be used together.
+directio               (For file-backed mounts) Use direct I/O to access backing
+                       files, and asynchronous I/O will be enabled if supported.
 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.