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>

fs/btrfs/file-item.c

@@ -450,6 +450,19 @@ blk_status_t btrfs_lookup_bio_sums(struct btrfs_bio *bbio)
 	return ret;
 }
 
+/*
+ * Search for checksums for a given logical range.
+ *
+ * @root:		The root where to look for checksums.
+ * @start:		Logical address of target checksum range.
+ * @end:		End offset (inclusive) of the target checksum range.
+ * @list:		List for adding each checksum that was found.
+ * @search_commit:	Indicate if the commit root of the @root should be used
+ *			for the search.
+ * @nowait:		Indicate if the search must be non-blocking or not.
+ *
+ * Return < 0 on error and 0 on success.
+ */
 int btrfs_lookup_csums_list(struct btrfs_root *root, u64 start, u64 end,
 			    struct list_head *list, int search_commit,
 			    bool nowait)