expiry: clarify late expiry events

[oliver-sanders]

• Addresses expiry: clarify late expiry cylc-doc#974
• Add some text to the examples section explaining when expiry happens.

Goes with cylc/cylc-doc#974

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 pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[hjoliver]

Suggested change

	If you need something in your workflow to happen at a configured time, use
	:term:`clock triggers <clock trigger>` rather than triggering off of the
	``:expired`` output.

I'd suggest not putting this here, unless you have evidence that users try to use expiry for this purpose? Triggering off of an expiry event seems an odd way to attempt anything other than respond to the expiry itself (i.e. "this task expired, it's not going to run, so do this instead").

Besides, clock triggers actually have a similar "lateness" problem to expiry - they aren't checked until the dependent task enters the active window, so clock triggers make things happen after, not at, a configured time.