Merge pull request #8277 from miri64/doc/fix/must-not-may-not-wording

core/include/lifo.h

sys/include/net/gnrc/ipv6/nib/nc.h

sys/include/net/gnrc/ipv6/nib/pl.h

sys/include/net/gnrc/ndp.h

sys/include/net/gnrc/netif/internal.h

sys/include/net/gnrc/pktqueue.h

sys/include/net/gnrc/priority_pktqueue.h

sys/include/net/ndp.h

sys/include/net/sixlowpan.h

sys/include/net/sixlowpan/nd.h

sys/include/net/sock/udp.h

sys/net/gnrc/network_layer/ipv6/nib/_nib-arsm.h

sys/net/gnrc/network_layer/ipv6/nib/_nib-internal.h