net/dhcpv6 : Update documentation
net/dhcpv6 : Update documentation

Add configuration 'CONFIG_DHCPV6_CLIENT_PFX_LEASE_MAX' to
configuration group

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