doc: Explain light-handed markup preference a bit better

We're still pretty far away from anything like a consensus, but
there's clearly a lot of people who prefer an as-light as possible
approach to converting existing .txt files to .rst. Make sure this is
properly taken into account and clear.

Motivated by discussions with Peter and Christoph and others.

Cc: Christoph Hellwig <hch@infradead.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>