pf.conf.5: rework the text on mtu and mss

According to some notes from sthen;
ok sthen

Obtained from:	OpenBSD, jmc <jmc@openbsd.org>, 7f29e7e980
Sponsored by:	Rubicon Communications, LLC ("Netgate")

share/man/man5/pf.conf.5

@@ -27,7 +27,7 @@
 .\" ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 .\" POSSIBILITY OF SUCH DAMAGE.
 .\"
-.Dd July 9, 2025
+.Dd July 18, 2025
 .Dt PF.CONF 5
 .Os
 .Sh NAME
@@ -855,7 +855,15 @@ modifier to ensure unique IP identifiers.
 .It Ar min-ttl Aq Ar number
 Enforces a minimum TTL for matching IP packets.
 .It Ar max-mss Aq Ar number
-Enforces a maximum MSS for matching TCP packets.
+Reduces the maximum segment size (MSS)
+on TCP SYN packets to be no greater than
+.Ar number .
+This is sometimes required in scenarios where the two endpoints
+of a TCP connection are not able to carry similar sized packets
+and the resulting mismatch can lead to packet fragmentation or loss.
+Note that setting the MSS this way can have undesirable effects,
+such as interfering with the OS detection features of
+.Xr pf 4 .
 .It Xo Ar set-tos Aq Ar string
 .No \*(Ba Aq Ar number
 .Xc