Document `:offset`

[aphillips]

We made a change to remove the :math function and add a function called :offset instead. This feature provides support for the MF1 feature of plural offsets (hence the name). See unicode-org/message-format-wg#1073

[lucacasonato]

:math is not documented here at all yet (neither is :offset).

It would be great if someone who actively attends the group meetings could update the docs!