Clarify Cron scheduling information

[Niksko]

Currently, the PodChaos docs page links off to https://pkg.go.dev/github.com/robfig/cron?utm_source=godoc for information about the cron format. There are a couple of issues with this:

• The actual cron format needs to be documented. I think the cron spec for Chaos Mesh starts with minutes and includes the optional 'day of week' field, but this should be documented.
• Document that all schedules are in UTC. This limitation is quite important, because it means something like setting up chaos only during business hours becomes non trivial, because cron schedules can't 'wrap' over midnight, so 9am to 5pm in your local time might have to be configured as two separate scheduled bits of chaos when converted to UTC.

[g1eny0ung]

Thanks for your advice! Currently, we move this part to https://chaos-mesh.org/docs/define-scheduling-rules/. I think these two issues have been already solved.