Check documentation of what --icp can accept.

[wxtim]

Describe the bug
On a quick look for #4554 I couldn't find definitive docs of what --icp can accept; and the code from that PR suggests that it can accept offsets as well as time points.

Investigate and make sure that this is documented in the CLI help (cylc/flow/scripts/...), and, if necessary the cylc-doc repo.

Pull requests welcome!
This is an Open Source project - please consider contributing a bug fix
yourself (please read CONTRIBUTING.md before starting any work though).