typofix
various updates

- remove norm sections on atomic frags (handled by fernando's draft)
- remove norm section on flow labels as there is some controversy there,
  leave the behaviour to rfc6145 (and possible updates)
- improve language, fix typos, rephrase sentences for clarity
- improve guidance sections on atomic frags and v4/v6 pmtu differences
- improve some points in the v4+nat64 section

Thanks Fred Baker, Brian E Carpenter, and Knut A. Syed!