Update docs/usage for 'plink -shareexists'.

unix/uxplink.c

@@ -581,6 +581,8 @@ static void usage(void)
     printf("  -N        don't start a shell/command (SSH-2 only)\n");
     printf("  -nc host:port\n");
     printf("            open tunnel in place of session (SSH-2 only)\n");
+    printf("  -shareexists\n");
+    printf("            test whether a connection-sharing upstream exists\n");
     exit(1);
 }