for --sort you can combine some items together by \',', just like
--sort=+ogen,-gen,path,rootid.
-*set-default* <id> <path>::
-Set the subvolume of the filesystem <path> which is mounted as
-default.
+*set-default* [<subvolume>|<id> <path>]::
+Set the default subvolume of the (mounted) filesystem.
+
-The subvolume is identified by <id>, which is returned by the *subvolume list*
-command.
+There are two ways how to specify the subvolume, by <id> or by the <subvolume>
+path.
+The id can be obtained from *btrfs subvolume list*, *btrfs subvolume show* or
+*btrfs inspect-internal rootid*.
*show* <path>::
Show information of a given subvolume in the <path>.