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 ce27ec6064 ("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>

Documentation/admin-guide/sysctl/net.rst

@@ -391,6 +391,18 @@ GRO has decided not to coalesce, it is placed on a per-NAPI list. This
 list is then passed to the stack when the number of segments reaches the
 gro_normal_batch limit.
 
+high_order_alloc_disable
+------------------------
+
+By default the allocator for page frags tries to use high order pages (order-3
+on x86). While the default behavior gives good results in most cases, some users
+might have hit a contention in page allocations/freeing. This was especially
+true on older kernels (< 5.14) when high-order pages were not stored on per-cpu
+lists. This allows to opt-in for order-0 allocation instead but is now mostly of
+historical importance.
+
+Default: 0
+
 2. /proc/sys/net/unix - Parameters for Unix domain sockets
 ----------------------------------------------------------