document task outputs for retries

[oliver-sanders]

Tasks will only enter the failed state after all execution retry delays have been exhausted. It's the same deal with submit-failed and submission retry delays.

See also: https://cylc.discourse.group/t/fail-qualifier-with-execution-retries/571/2

Retry info is spread throughout the docs and not especially well inter-linked: https://cylc.github.io/cylc-doc/stable/html/search.html?q=retries&check_keywords=yes&area=default

I did eventually find something on this in this section:

https://cylc.github.io/cylc-doc/stable/html/user-guide/writing-workflows/runtime.html#task-retry-on-failure

• We should gather the retry docs where appropriate.
• We should ensure the impact of retries on task outputs is clearly documented.
• We should inter-link the docs, tutorial and reference sections for retries.