docs: fix padding descriptions

docs: fix padding descriptions

Mentioning just answers is misleading.  Padding is very important
for queries as well; for us that applies during forwarding over TLS.

Also describe /tls/auto_discovery as experimental in the
configuration  schema.  It's a rather dead experiment from long ago.
I hope it can't be confused with more recent things like
https://datatracker.ietf.org/doc/draft-ietf-dprive-unilateral-probing/