Replace raw SQL suggestion in TIME_ZONE docs

[Jacob Walls]

Our docs counsel avoiding raw SQL usage where the ORM can do. We have old advice in the TIME_ZONE setting advising to use "raw SQL" when local time conversions are needed. I think instead we should suggest using a lightweight Func wrapping ​timezone() to accomplish the same thing:

​Current text:

  Consider converting to local time explicitly with ``AT TIME ZONE`` in raw SQL
  queries instead of setting the ``TIME_ZONE`` option.

[Natalia Bidart]

Spot on Jacob, thank you! Are we targeting something like this:

    class TimeZone(Func):
        function = "timezone"
        output_field = DateTimeField()

Ideally we would also include a minimal usage example and have a cross-reference to the Func docs and a note that this is PostgreSQL-specific.

[Jacob Walls]

Exactly, assuming a contributor validates that it works. The context for this remark about the setting is postgres-specific anyway, but totally agree about mentioning the snippet is PostgreSQL-specific.