Documentation: fix outdated interpretation of ip_no_pmtu_disc

The updating way of pmtu has changed, but documentation is still in the
old way. So this patch updates the interpretation of ip_no_pmtu_disc and
min_pmtu.

See commit 28d35bc ("net: ipv4: don't let PMTU updates increase
route MTU")

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: xu xin <xu.xin16@zte.com.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.rst

@@ -25,7 +25,8 @@ ip_default_ttl - INTEGER
 ip_no_pmtu_disc - INTEGER
 	Disable Path MTU Discovery. If enabled in mode 1 and a
 	fragmentation-required ICMP is received, the PMTU to this
-	destination will be set to min_pmtu (see below). You will need
+	destination will be set to the smallest of the old MTU to
+	this destination and min_pmtu (see below). You will need
 	to raise min_pmtu to the smallest interface MTU on your system
 	manually if you want to avoid locally generated fragments.
 
@@ -49,7 +50,8 @@ ip_no_pmtu_disc - INTEGER
 	Default: FALSE
 
 min_pmtu - INTEGER
-	default 552 - minimum discovered Path MTU
+	default 552 - minimum Path MTU. Unless this is changed mannually,
+	each cached pmtu will never be lower than this setting.
 
 ip_forward_use_pmtu - BOOLEAN
 	By default we don't trust protocol path MTUs while forwarding