fs: improve comments for writeback_single_inode()

Some comments for writeback_single_inode() and
__writeback_single_inode() are outdated or not very helpful, especially
with regards to writeback list handling.  Update them.

Link: https://lore.kernel.org/r/20210112190253.64307-10-ebiggers@kernel.org


Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jan Kara <jack@suse.cz>