correct RT::Date->Timezone argument error message

This method is not a setter.
correct RT::Date->Timezone documentation

  - There is no special handling for a "utc" context.

  - There is no default value for $context.

  - "UTC" may be still returned in cases where one of the server's and
    user's timezones is set.
Merge branch '4.2/date-timezone-docs' into 4.2-trunk