net: udp: correct the document for udp_mem

udp_mem is a vector of 3 INTEGERs, which is used to limit the number of
pages allowed for queueing by all UDP sockets.

However, sk_has_memory_pressure() in __sk_mem_raise_allocated() always
return false for udp, as memory pressure is not supported by udp, which
means that __sk_mem_raise_allocated() will fail once pages allocated
for udp socket exceeds udp_mem[0].

Therefor, udp_mem[0] is the only one that limit the number of pages.
However, the document of udp_mem just express that udp_mem[2] is the
limitation. So, just fix it.
Signed-off-by: Menglong Dong <imagedong@tencent.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.rst

@@ -1004,13 +1004,11 @@ udp_l3mdev_accept - BOOLEAN
 udp_mem - vector of 3 INTEGERs: min, pressure, max
 	Number of pages allowed for queueing by all UDP sockets.
 
-	min: Below this number of pages UDP is not bothered about its
-	memory appetite. When amount of memory allocated by UDP exceeds
-	this number, UDP starts to moderate memory usage.
+	min: Number of pages allowed for queueing by all UDP sockets.
 
 	pressure: This value was introduced to follow format of tcp_mem.
 
-	max: Number of pages allowed for queueing by all UDP sockets.
+	max: This value was introduced to follow format of tcp_mem.
 
 	Default is calculated at boot time from amount of available memory.