Documentation/network: reword kernel version reference

BugLink: https://bugs.launchpad.net/bugs/1818813

It seemed odd to say "since 4.17" in a 4.4 kernel. Consider
rewording the reference to indicate where in the stable series
it was introduced as well as where it originated.
Signed-off-by: Mark Rustad <mrustad@gmail.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

Documentation/networking/ip-sysctl.txt

@@ -116,7 +116,7 @@ ipfrag_high_thresh - LONG INTEGER
 	Maximum memory used to reassemble IP fragments.
 
 ipfrag_low_thresh - LONG INTEGER
-	(Obsolete since linux-4.17)
+	(Obsolete since linux-4.4.174, backported from linux-4.17)
 	Maximum memory used to reassemble IP fragments before the kernel
 	begins to remove incomplete fragment queues to free up resources.
 	The kernel still accepts new fragments for defragmentation.