Unify tz doc
These specifically mention OlsonNames: https://github.com/USGS-R/dataRetrieval/blob/99422035df9307101452cf57d82ac4c6013058c4/R/readWQPdata.R
The rest don't....I lean towards holding the users hand a bit and listing out North American timezones specifically. I like:
#' @param tz character to set timezone attribute of dateTime. Default is UTC (properly accounting for daylight savings times based on the data's provided tz_cd column).
#' Possible values include "America/New_York","America/Chicago", "America/Denver","America/Los_Angeles",
#' "America/Anchorage","America/Honolulu", as well as the following which do not use daylight savings time: "America/Jamaica","America/Managua","America/Phoenix", and "America/Metlakatla"
Also add in the "Details" section a reference to OlsonNames.