Time: mention IANA TZ Database in help text of tztime

Because I have more than one commit at the time of creating pull request, commit text doesn't come here automatically.

From b8193e0c885fc867a0acec3d74891f90deea53d7:
> https://www.iana.org/time-zones uses the spelling IANA Time Zone Database, so I use it too.

From 0ce8ef829935f07607165e6d07742367c93ce5ca:
> I translated IANA Time Zone Database too, because I tried googling for my translation and the first results are about correct thing (even if Google first suggests wrong spelling which (when clicked) suggests two other wrong spellings…