Update patterns of allowable subdomains for multiple environments

This commit clarifies that given you have a domain of myservice.service.gov.uk and you have multiple environments, if you want to refer to your public site in those environments, you may use either of the following schemes:

- Scheme 1: hyphenated suffixes
    www-dev.myservice.service.gov.uk
    www-preview.myservice.service.gov.uk
    www.myservice.service.gov.uk

- Scheme 2: subdomains
    www.dev.myservice.service.gov.uk
    www.preview.myservice.service.gov.uk
    www.myservice.service.gov.uk

I've run this past @jystewart and @ejhp before submitting, because I want to use the latter scheme for Performance Platform and have a valid technical reason to need it (SSL wildcards).