docs: better description for the `cron` attribute of `coder_script`

[jatcod3r]

Was playing around with the cron attribute, and found out that it parses expressions in seconds minutes hours ... instead of minutes hours .... I can see here that the Coder Agent is configured to run this way

Would be nice to get this documented as I was expecting my script to run every 5 minutes with this cron expression, but instead it was running every 5 seconds. It also conflicts with my understanding of other cron settings on Coder such as CODER_QUIET_HOURS_DEFAULT_SCHEDULE, which ingests an expression following the minutes hours ... format.

[matifali]

I will consider this a bug if we are not following the UNIX cron format

We can address this with document changes for now.

[matifali]

@jatcod3r would you like to submit a PR addressing this? I can help if you get into any issues. Thanks.