Documentation: add a description for net.core.high_order_alloc_disable

Documentation: add a description for net.core.high_order_alloc_disable

A description is missing for the net.core.high_order_alloc_disable
option in admin-guide/sysctl/net.rst ; add it. The above sysctl option
was introduced by commit bbb0b6b1a2af ("net: add high_order_alloc_disable
sysctl/static key").

Thanks to Eric for running again the benchmark cited in the above
commit, showing this knob is now mostly of historical importance.

Signed-off-by: Antoine Tenart <atenart@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20220707080245.180525-1-atenart@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>