gui: warn over unusual uses

[oliver-sanders]

There are some use patterns that users likely don't intend but don't realise they are following.

Make them aware with some quick prompts.

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.

[oliver-sanders]

(template solution, not assigning myself to finish just yet, feel free to grab this PR).

[wxtim]

Over nested>?

Suggested change

	if '--help-all' not in set(sys.argv):
	if hub_url:
	if '--help-all' not in set(sys.argv) and hub_url:

[wxtim]

Seems conceptually reasonable.

[MetRonnie]

Why remove --help from this line?

[oliver-sanders]

(soz, was supposed to be draft)