ipv6: Document that max_size sysctl is deprecated

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2164260
Upstream Status: net.git commit 695a376

commit 695a376
Author: Jon Maxwell <jmaxwell37@gmail.com>
Date:   Sat Jan 21 10:23:31 2023 +1100

    ipv6: Document that max_size sysctl is deprecated

    v4: fix deprecated typo.

    Document that max_size is deprecated due to:

    commit af6d103 ("ipv6: remove max_size check inline with ipv4")

    Signed-off-by: Jon Maxwell <jmaxwell37@gmail.com>
    Link: https://lore.kernel.org/r/20230120232331.1273881-1-jmaxwell37@gmail.com
    Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Signed-off-by: Hangbin Liu <haliu@redhat.com>

Author: Hangbin Liu

Documentation/networking/ip-sysctl.rst

@@ -154,6 +154,9 @@ route/max_size - INTEGER
 	From linux kernel 3.6 onwards, this is deprecated for ipv4
 	as route cache is no longer used.
 
+	From linux kernel 6.3 onwards, this is deprecated for ipv6
+	as garbage collection manages cached route entries.
+
 neigh/default/gc_thresh1 - INTEGER
 	Minimum number of entries to keep.  Garbage collector will not
 	purge entries if there are fewer than this number.