Update padding text based on Alex's review comments.

[saradickinson]

Fix a couple of typos.

[saradickinson]

Proposed text for https://github.com/huitema/dnsoquic/issues/129

[amankin-s]

@saradickinson your padding text/references work for me. Hope they are good for @alex-nicat too

[alex-nicat]

Hello everyone - the changes look good to me, as it works around the fact that padding policy recommendations might change with operational experience over time (and avoid the downref).

[saradickinson]

Thanks @alex-nicat!