add comments to javac/util/Convert.java

From Peter Ahe:

I just noticed something that may not be obvious.

The JVMS defines its own version of the UTF-8 format so that it is
contains no zero bytes (modified UTF-8). This is not actually the same
as Charset.forName("UTF-8"). See also:
http://java.sun.com/javase/6/docs/api/java/io/DataInput.html#modified-utf-8

You might want to explicitly document this in javac/util/Convert.java.