IPFW: Warning about logging

[trhodes]

Summary

If someone enables logging, as noted by the instructions, they need to ensure rules have the "log" option. If this is missed, a user may not realize it's needed even after reading the manual page and notes below. For example, I believed, incorrectly, that just turning on logging would have the "log" effect and cleaned it up from my rules. It turned out, even after you enable logging, the "log" option is still needed. By noting that in a warning, we can prevent users from believing it's automatic.

Test Plan

Run a "make lint" prior to commit.

[bcr]

Looks good, approved!