rtt_rtc: clarify documentation about epoch
rtt_rtc: clarify documentation about epoch

`epoch` in this context refers to `RIOT_EPOCH`, not Unix Epoch.
Clarify the documentation to avoid confusion.

miri64/RIOT,jasonatran/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,kYc0o/RIOT,miri64/RIOT,jasonatran/RIOT,ant9000/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,kaspar030/RIOT,jasonatran/RIOT,miri64/RIOT,ant9000/RIOT,ant9000/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,kaspar030/RIOT,kYc0o/RIOT,OlegHahm/RIOT,miri64/RIOT,RIOT-OS/RIOT,ant9000/RIOT,ant9000/RIOT,OlegHahm/RIOT,kaspar030/RIOT,kYc0o/RIOT,kYc0o/RIOT,miri64/RIOT,jasonatran/RIOT