This is a light restructuring of the tz documentation because the automodule-generated stuff was not great to read.

The downside of using all these explicit directives is that if we forget to add something to tz.rst it won't get added to the documentation, but given how much stuff we've forgotten to document anyway we should probably be improving discipline on that anyway. At least this way we don't have to wait until the build is done to figure out if something is missing.