expiry: clarify late expiry

[oliver-sanders]

Addresses #699

There was already a note in this documentation section, so not much to do here, but have clarified / corrected the text slightly.

Goes with cylc/cylc-flow#7505

Requirements check-list

• I have read CONTRIBUTING.md and added my name as a Code Contributor.

[hjoliver]

Maybe say expiry can only be detected in active waiting tasks. Other (future tasks) can expire, but they won't until they become active.

Suggested change

	Only :term:`active <active task>` waiting tasks can expire, submitted or
	running tasks will not be killed, and tasks with a :term:`final status` will
	not be removed if they pass their configured ``clock-expire`` time.
	Expiry can only be detected once a task has entered the active window of the
	workflow. Submitted, running, or final status tasks cannot expire (and will
	not be killed or removed if they pass their configured ``clock-expire`` time).

[hjoliver]

(It could also be noted that the purpose of expiry to is cancel a task before it runs, so in light of that purpose it is OK if expiry is detected "late" relative to the configured clock time, so long as it is detected before the task runs ... but I think that's been said elsewhere already).

[oliver-sanders]

In Cylc 7 we had clock-trigger and clock-expire side-by-side, and both worked, kinda similarly. Due to SoS, the next instance of each task was spawned ahead of time, activating expiry detection.

With Cylc 8, expiry in future cycles is more adversely delayed by current cycles due to delayed spawning and as a result, this was once reported as a Cylc 8 migration issue.

As a result, in the optional output proposal, we added point 9 to document that expiry events are not instantaneous - the issue got lost for a while, I picked it up last week: https://cylc.github.io/cylc-admin/proposal-optional-output-extension.html#proposal

Example Cylc 7 use cases might look like this:

# poll for live data
get_live_data

# fallback to archive data after the expire time
# (we don't want this to wait longer than necessary)
get_live_data:expired => get_archive_data

# carry on
get_live_data | get_archive_data => run

# for this example to make sense standalone
get_live_data[-P1D] => get_live_data

With Cylc 8, the way to achieve this would likely be to use a clock-trigger to detect expiry, rather than a clock-expire:

get_live_data[-P1D] => get_live_data
@live_data_expiry => !get_live_data & get_archive_data
get_live_data => !get_archive_data
get_live_data | get_archive_data => run