netlink.7: Add a comment on 8192 buffer size in example code

Reported-by: Rick Jones <rick.jones2@hpe.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man7/netlink.7

@@ -533,7 +533,8 @@ And the last example is about reading netlink message.
 .in +4n
 .nf
 int len;
-char buf[8192];
+char buf[8192];     /* 8192 to avoid message truncation on
+                       platforms with page size > 4096 */
 struct iovec iov = { buf, sizeof(buf) };
 struct sockaddr_nl sa;
 struct msghdr msg;