pkg/doc.txt: improve rendering of source location documentation
pkg/doc.txt: improve rendering of source location documentation

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