doc: document auto-restart "force mode"

[oliver-sanders]

This used to be documented but got lost for some reason.

Add some docs back, I'll follow this with a cylc-doc PR in the future.

Don't merge before #6623.

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 if present).
• Tests are included (or explain why tests are not needed).
• Changelog entry included if this is a change that can affect users
• Cylc-Doc soon, soon.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[oliver-sanders]

Unrelated syntax error.

[oliver-sanders]

There's a bigger issue with this feature, closing this - #6623