[DOC] Extend FROM_UNIXTIME() doc with Timezone offset behaviour.

FROM_UNIXTIME() accepts a format parameter that is a string that represents how this function should format its output timestamp. This format parameter can contain a timezone offset, however even if we provide a TZ offset in the format parameter it won't be included in the result.

The reason is that Impala stores Timestamp without timezone in UTC and has no information of the timezone offset.

I think it would be nice to clarify this in the docs so that the users won't expect to get specific timezone offsets from this function as a result.