Clarify documentation for `-syslog` option

https://developer.hashicorp.com/consul/docs/agent/config/cli-flags#_log_file
> [-syslog](https://developer.hashicorp.com/consul/docs/agent/config/cli-flags#_syslog) - This flag enables logging to syslog. This is only supported on Linux and macOS. It will result in an error if provided on Windows.

From the documentation it is not clear: will this option disable logging to file, or it will log both: to file and to syslog.

Thanks.

Start with the linked Consul CLI flags documentation, especially the -syslog and -log-file sections. Determine whether -syslog replaces file logging or enables both, then update the page so this behavior and the platform limitation are explicit.

Written by the indexing model from the issue text.