btrfs: add function comment to btrfs_lookup_csums_list()

btrfs: add function comment to btrfs_lookup_csums_list()

Add a function comment to btrfs_lookup_csums_list() to document it.
With another upcoming change its parameter list and return value will be
less obvious. So add the documentation now so that it can be updated where
needed later.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>