Add clarifying text for Time Zone select when configuring inputs

## What?

Add text clarifying and making it explicit what exactly configuring a Time Zone on an input does and what value should be set.

Current state: Default time zone used when no timezone detected

Suggested:

> Configure this time zone to match the time zone of the devices sending log data to graylog. For example, if your source device is configured to use UTC, you should configure UTC here.
>
> If this is not configured, the time zone of the Operating System where graylog is installed will be used.

I believe this would apply to the following inputs (but i may be forgetting some?)

* Syslog
* Palo Alto

## Why?

A user misconfigured this by mistake due to the ambiguity.

note: this was timezone config for syslog was introduced via https://github.com/Graylog2/graylog2-server/issues/3853