btrfs-progs: prop set: enhance error message when changing subvolume from ro->rw

The error message for ro->rw is not clear enough, add more context for
the received_uuid and send and point to the manual page that explains
more.

Asked at:
https://www.reddit.com/r/btrfs/comments/1bkqor2/received_uuid_error_after_btrfs_send/

[ci skip]

Issue: #763
Signed-off-by: David Sterba <dsterba@suse.com>

cmds/property.c

@@ -109,7 +109,9 @@ static int prop_read_only(enum prop_object_type type,
 				do_clear_received_uuid = true;
 			} else {
 				error(
-"cannot flip ro->rw with received_uuid set, use force if you really want that");
+"cannot flip ro->rw with received_uuid set, use force option -f if you really want unset the read-only status."
+" The value of received_uuid is used for incremental send, consider making a snapshot instead."
+" Read more at btrfs-subvolume(8) and Subvolume flags.");
 				return -EPERM;
 			}
 		}