pf.conf.5: sync documentation with code on the matter of max state limit behavior

When one of the state limits is reached, further packets that would
create state are dropped, until existing states time out. Discussed
with mcbride, ok henning, jmc

Obtained from: OpenBSD, mikeb <mikeb@openbsd.org>, 677ed08ce1
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D46932