ipv6: Document that max_size sysctl is deprecated

v4: fix deprecated typo.

Document that max_size is deprecated due to:

commit af6d1034 ("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>