common: document the reasons behind NERR_RPC_ERROR and NERR_UNKNOWN_ERROR
common: document the reasons behind NERR_RPC_ERROR and NERR_UNKNOWN_ERROR

Signed-off-by: Josef 'Jeff' Sipek <a620f141433c70aa9e43778305a6c68cbfadfb25@josefsipek.net>

jeffpc/nx01