Mention no-df bug in upgrade46.html?

[Matthew Dempsky]

Just since upgrade46.html mentions the new syntax for packet scrubbing
and specifically describes the behavior of the no-df flag, perhaps it
should mention that the 4.6-release (and 4.6-stable?) kernel uses the
opposite semantics for this setting?  (See r1.120 of
sys/net/pf_norm.c.)