Manual 802.15.4 variable descriptions

[boulis]

Follow this thread and make sure that the manual describes the terms correctly.

https://groups.google.com/forum/?utm_medium=email&utm_source=footer#!searchin/castalia-simulator/band/castalia-simulator/BcRgTWLIi2I/qkil5BJwjCIJ

[boulis]

Corrections made in the manual with the 5241f75 commit close the issue