Clarify spec for qualifiers

[pombredanne]

No description provided.

[johnmhoran]

Relevant issues and PRs include the following (from the "encoding" Google Sheet):

• Clarifications, in particular about version encoding #35
• Percent encoding spec and : and / #39
• Inconsistency in the description of the checksum qualifier. #73
• docs(types): oci type repositry_url must be percent encoding #150
• Revert "Fix missing percent-encoding in purl example with repository_url" #184
• feat(test-suite-data): add test case for checksum qualifier #207
• define handling of plus and space #261
• Add RFC 2119/8174 note and convert terms to uppercase as needed #373

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

2025-02-06 Note: after discussion at yesterday's biweekly PURL community meeting, we are back to using "not"/"NOT" when that most clearly and succinctly expresses the intent.