doc: networking: ip-sysctl: Document conf/all/disable_ipv6 and conf/default/disable_ipv6

This patch adds documentation for sysctl conf/all/disable_ipv6 and
conf/default/disable_ipv6 settings which is currently missing.

Signed-off-by: Pali Rohár <pali@kernel.org>
Link: https://lore.kernel.org/r/20210121150244.20483-1-pali@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>