btrfs-progs: docs: clarify how btrfs check works with replicas

- Btrfs-check does device assembly automatically
  Thus no difference when specifying different devices of the same
  filesystem

- Btrfs-check automatically choose good metadata
  Thus as long as there is any good mirror for metadata, it will not
  report error for that repariable metadata.

Signed-off-by: Colin Snover <[email protected]>
[ Add an SoB line, remove the scrub recommendation as btrfs-check is
  supposed to choose the good mirror ]
Signed-off-by: Qu Wenruo <[email protected]>

Documentation/btrfs-check.rst

@@ -19,6 +19,13 @@ by the option *--readonly*.
 
 :command:`btrfsck` is an alias of :command:`btrfs check` command and is now deprecated.
 
+.. note::
+   Even though the filesystem checker requires a device argument, it scans for all
+   devices belonging to the same filesystem, thus it should not cause a difference
+   using different devices of the same filesystem.
+   Furthermore `btrfs check` will automatically choose the good mirror, thus as long
+   as there is a good copy for metadata, it will not report such case as an error.
+
 .. warning::
    Do not use *--repair* unless you are advised to do so by a developer
    or an experienced user, and then only after having accepted that no *fsck*