RFR: Update build documentation to reflect compiler upgrades at Oracle

[Erik Joelsson]

The build documentation needs to be updated to reflect the compiler 
updates that took place at Oracle for JDK 11.

Bug: https://bugs.openjdk.java.net/browse/JDK-8208096
Webrev: http://cr.openjdk.java.net/~erikj/8208096/webrev.01/

/Erik