Enhancements to `dateformat` & `archive_dateformat` pophelps

[petecooper]

We need to revise the help text for dateformat & archive_dateformat, given the changes between 4.8.7 and 4.8.8.

4.8.7

4.8.8

Please can someone create and tag this with a 4.8.8 milestone as I don't appear to have the privs. Thanks.

[petecooper]

Paging textpattern/textpattern#1720