Document advanced agent options formats and validations.

Goal

Extracted from #5222 (see https://github.com/fleetdm/fleet/issues/5222#issuecomment-1251096271).

The various Agent Options settings are now validated based on their data type, but reading the osquery documentation about those settings indicates that there are a number of settings that have restrictions that go beyond their data type. That is, a value may be correct for the required data type of a setting, but may still be invalid for osquery. While we don't currently validate those extra restrictions (it might be very hard to cover them all and keep them up to date on each release), we should at least document them as much as we can, as clear as we can.

Some examples of such restrictions:

https://osquery.readthedocs.io/en/stable/deployment/configuration/#schedule : the query should be valid SQL, the platform should represent valid platform labels and be comma-separated if it has more than one, shard is meant to be a percentage between 1-100.
https://osquery.readthedocs.io/en/stable/deployment/configuration/#yara : the file_paths in the YARA section must match those in the top-level file_paths key, it must also refer to a valid signature name from the signatures array, the signatures must be valid file paths.
https://osquery.readthedocs.io/en/stable/deployment/configuration/#prometheus : the urls must be valid urls.
https://osquery.readthedocs.io/en/stable/deployment/configuration/#views : the keys must be valid osquery identifiers for tables, the value must be a valid SQL query.
https://osquery.readthedocs.io/en/stable/deployment/configuration/#decorator-queries : the interval section's keys must be numbers (as strings), and those numbers must be divisible by 60 as they represent minute intervals. "Anything not divisible by 60 will generate a warning, and will not run."
https://osquery.readthedocs.io/en/stable/deployment/file-integrity-monitoring/: some FIM options (such as exclude_paths) are only mentioned in this FIM-specific doc page, and not in the general configuration page.

As you can see, there are many such restrictions that can be found.

How?

Frontend

No impact on frontend, purely a documentation change.

Backend

No impact on backend, purely a documentation change.

fleetdm / fleet