Polina Goltsman issue 4

"old:
      directly between nodes.  TLS [RFC5246] and DTLS [RFC6347] are the
      currently defined "link layer" protocols used by RELOAD for hop-
new:
      currently defined "overlay link layer" protocols used by RELOAD
for hop-

avoids confusion with the classic ISO/OSI link layer (layer 2)"

- As suggested.