net/netopt: clarify documentation of NETOPT_OQPSK_RATE
net/netopt: clarify documentation of NETOPT_OQPSK_RATE

802.15.4 only specifies one rate of 250 kbit/s for O-QPSK.
Everything else is a proprietary extension.

kYc0o/RIOT,OlegHahm/RIOT,miri64/RIOT,basilfx/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,miri64/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,miri64/RIOT,authmillenon/RIOT,ant9000/RIOT,kaspar030/RIOT,ant9000/RIOT,miri64/RIOT,kaspar030/RIOT,OlegHahm/RIOT,kYc0o/RIOT,miri64/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,kYc0o/RIOT,OTAkeys/RIOT,authmillenon/RIOT,authmillenon/RIOT,authmillenon/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,ant9000/RIOT,basilfx/RIOT,kYc0o/RIOT,basilfx/RIOT,jasonatran/RIOT,ant9000/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,jasonatran/RIOT,authmillenon/RIOT,jasonatran/RIOT,basilfx/RIOT,kYc0o/RIOT,basilfx/RIOT,RIOT-OS/RIOT,ant9000/RIOT,kaspar030/RIOT,kaspar030/RIOT,OlegHahm/RIOT,OTAkeys/RIOT