Documentation: fix udp_wmem_min in ip-sysctl.rst

UDP doesn't support tx memory accounting, and sysctl udp_wmem_min
is not really used anywhere. So we should fix the description in
ip-sysctl.rst accordingly.

Fixes: 95766fff ("[UDP]: Add memory accounting.")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Link: https://lore.kernel.org/r/c880a963d9b1fb5f442ae3c9e4dfa70d45296a16.1658167019.git.lucien.xin@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/networking/ip-sysctl.rst

@@ -1052,11 +1052,7 @@ udp_rmem_min - INTEGER
 	Default: 4K
 
 udp_wmem_min - INTEGER
-	Minimal size of send buffer used by UDP sockets in moderation.
-	Each UDP socket is able to use the size for sending data, even if
-	total pages of UDP sockets exceed udp_mem pressure. The unit is byte.
-
-	Default: 4K
+	UDP does not have tx memory accounting and this tunable has no effect.
 
 RAW variables
 =============