Add descriptions to timezone functions
Add descriptions to timezone functions

These are quite complex things and benefit from having better descriptions.

Note, we weren't quite happy with the names of the functions.

`aware_gmt_datetime` should really be `aware_london_datetime` and
the other two functions could have more verbose names (mentioning
that they convert from naive to naive) but we decided not to change
these as will involve lots of changes around the codebase.

alphagov/notifications-utils