Add a bugs section to pflog man page

FreeBSD does not set the pid field in the pfloghdr struct. This field is
populated on other platforms, document this to save people from trying
to use this field.

Event: Aberdeen hackathon 2019
Reviewed by: kp, bcr, bz
Approved by: bz (mentor)
Differential Revision: https://reviews.freebsd.org/D19952