Improve DNS guidance

Many services are being sent to the Infrastructure Team without
either the correct information about DNS servers, or are expecting
GDS to host the DNS for their service.

This PR makes it much more explicit about how service.gov.uk DNS
management differs from direct.gov.uk management and what information
needs to be provided.
Various style and typo fixes
Merge pull request #267 from alphagov/improve-dns-guidance

Improve DNS guidance