Trigger and set: fix default flow assignment for n=0 tasks.

[hjoliver]

Close #6361

FIX

• by default (i.e., --flow option not used), existing (n=0) tasks should keep their existing flow numbers

Tidy

• remove redundant option checking, now done during command validation
• clarify command and schema help

NEW

• explicit use of --flow will merge the given flow numbers into existing (n=0) tasks
  (this seems sensible to me, and I can't think of a reason not to do it)

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.