improve tz documentation with better examples

[vincentvanhees]

• Currently the tz documentation uses "" as example, which is not helpful as this is not in the timezone database page https://en.wikipedia.org/wiki/List_of_tz_database_time_zones. We need to give a real example such as "Europe/Brussels" as currently some users think they need to specify GMT or CET, which do not account for day saving time.
• Similarly, we may need to clarify that timenum in the part5 time series output is UTC time, which means seconds since 1-1-1970. This does not account for DST and we may need to give examples of how to convert it to local time as I have encountered several users who complained that the times are wrong not realising that their interpretation of these timestamps is wrong.