[Security] Add hostname/FQDN documentation for filtering

[elasticmachine]

Original comment by @gmoskovicz:

See LINK REDACTED

xpack.security.transport.filter.allow seems to be needed the FQDN and no hostnames, regardless that the hostnames are resolving internally in each node. If you use the FQDN it will work. I tried to look around the repository but we don't have tests for this neither the documentation explicitly mentions that we need FQDN to work?

CC @jaymode

[elasticmachine]

Original comment by @jaymode:

This is pretty hard to test since you would need to setup DNS or some other way to resolve names. I think doc wise we can add something to that effect.

[elasticmachine]

Original comment by @gmoskovicz:

@jaymode docs will be a good idea. Do you know if FQDN is required, or this should work with hostnames?

[elasticmachine]

Original comment by @jaymode:

It is dependent on what reverse DNS resolves IIRC

[elasticmachine]

Original comment by @gmoskovicz:

If the hostname is setinside the allow filter, it doesn't work. If the FQDN or IP addressis set, it does work. So i think that FQDN is required. Hence my request 🙂

[jrodewig]

[docs issue triage]