docs: clarify that ADC directives are optional

[dbrattli]

Summary

• Clarify that ADC directives are optional.
• State that adopters may use only the directives relevant to non-obvious rationale, or none at all.

Why

The existing wording implied selective use, but an explicit statement avoids readers interpreting the four labels as a required checklist.

Validation

• npx --yes markdownlint-cli2@0.23.2 --config .markdownlint.jsonc "**/*.md"
• git diff --check
• Reviewed heading hierarchy with rg '^#{1,6} ' README.md AGENTS.md