btrfs: update documentation for a block group's bg_list member

Currently we are only documenting two uses of the bg_list member of a
block group, but there two more:

1) To track deleted block groups for discard purposes, introduced in
   commit e33e17ee ("btrfs: add missing discards when unpinning
   extents with -o discard");

2) To track block groups for automatic reclaim, introduced more recently
   by commit 18bb8bbf ("btrfs: zoned: automatically reclaim zones")

So document those two other use cases.
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/block-group.h

@@ -162,7 +162,14 @@ struct btrfs_block_group {
 	 */
 	struct list_head cluster_list;
 
-	/* For delayed block group creation or deletion of empty block groups */
+	/*
+	 * Used for several lists:
+	 *
+	 * 1) struct btrfs_fs_info::unused_bgs
+	 * 2) struct btrfs_fs_info::reclaim_bgs
+	 * 3) struct btrfs_transaction::deleted_bgs
+	 * 4) struct btrfs_trans_handle::new_bgs
+	 */
 	struct list_head bg_list;
 
 	/* For read-only block groups */