Possible documentation error with `codec` options that accept `UNITS_SINCE_EPOCH`.

[aimurphy]

In the settings.md and other reference materials in this repo, there are a few codec options that accept the special value UNITS_SINCE_EPOCH.

They have this text:

the unit and the epoch to use can be specified with `codec.unit` and `codec.timestamp`.

I think this is supposed to say epoch, not timestamp, because codec.timestamp doesn't set the epoch to use:

the unit and the epoch to use can be specified with `codec.unit` and `codec.epoch`.