RFR: JDK-8223726: j.l.c.MethodTypeDesc spec should contain precise assertions for one parameter's methods

[Vicente Romero]

Please review fix for [1] at [2], this is a simple fix that is just 
rewording the doc of a couple of methods at 
java.lang.constant.MethodTypeDesc,

Thanks,
Vicente

[1] https://bugs.openjdk.java.net/browse/JDK-8223726
[2] http://cr.openjdk.java.net/~vromero/8223726/webrev.00/