Clarify spec for type

[johnmhoran]

I'm beginning to work on the updates to

• PURL-SPECIFICATION.rst
• test-suite-data.json
• the new faq.rst -- initially created in the 'scheme'-focused PR 361, and will need to be created a new in each of the other such PRs until there's an instance merged into 'main that can then be updated going forward. ;-)

Relevant issues and PRs:

• Clarifications, in particular about version encoding #35

• Add RFC 2119/8174 note and convert terms to uppercase as needed #373

  • Add RFC 2119/8174 note and convert terms to uppercase as needed #373 (comment)

  • Add RFC 2119/8174 note and convert terms to uppercase as needed #373 (comment)

  • Add RFC 2119/8174 note and convert terms to uppercase as needed #373 (comment)

• Clarify encoding of colon between scheme and type #361

[johnmhoran]

We also need to

• review the relevant language etc. in PURL-SPECIFICATION.rst
• apply RFC 2119/8174 but replace the use of any "not"/"NOT" with affirmative language