btrfs-progs: doc: add description of missing and example, of device remove

btrfs-progs: doc: add description of missing and example, of device remove

This patch updates help/document of "btrfs device remove" in two points:

1. Add explanation of 'missing' for 'device remove'. This is only
written in wikipage currently.
(https://btrfs.wiki.kernel.org/index.php/Using_Btrfs_with_Multiple_Devices)

2. Add example of device removal in the man document. This is because
that explanation of "remove" says "See the example section below", but
there is no example of removal currently.

Signed-off-by: Tomohiro Misono <misono.tomohiro@jp.fujitsu.com>
Reviewed-by: Satoru Takeuchi <satoru.takeuchi@gmail.com>
[ move "" from the macro to help strings ]
Signed-off-by: David Sterba <dsterba@suse.com>