Improve config reference docs

[MetRonnie]

These changes close #4851

• Ensure config items shared by global and workflow configs have links to each other
• Centralised such items' descriptions and display the full description for the workflow item, whereas only the first paragraph for the global item
• Tidy up these docs a bit and fix the odd mistake

Would recommend a commit-by-commit review.

Requirements check-list

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg and conda-environment.yml.
• Does not need tests (just made sure docs build properly).
• No change log entry required (just a docs change).
• No documentation update required.