Document `ZonedDateTime.format_iso(tz=...)` in docstring

[injust]

The tz parameter is explained in https://whenever.readthedocs.io/en/latest/overview.html#formatting, but not in the docstring or https://whenever.readthedocs.io/en/latest/api.html#whenever.ZonedDateTime.format_iso.

IMO this is especially important because:

• The auto value is not self-explanatory
• Some parsers are not compatible with the IANA timezone identifier in square brackets

[ariebovenberg]

@injust thanks for posting. There are several docstrings that need attention, but you're right this one is particularly important. I'll see if I can fix this sooner rather than later