docs: discourage use of list tables

Our documentation encourages the use of list-table formats, but that advice
runs counter to the objective of keeping the plain-text documentation as
useful and readable as possible.  Turn that advice around the other way so
that people don't keep adding these tables.

Acked-by: Christoph Hellwig <hch@lst.de>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>