RFR: [16,test,docs] JDK-8247760 Improve documentation for javadoc.tester.JavadocTester

[Jonathan Gibbons]

Please review some updates to the doc comments in JavadocTester,
to better document the use of the starting/pass/fail methods.

-- Jon

JBS: https://bugs.openjdk.java.net/browse/JDK-8247760
Webrev: http://cr.openjdk.java.net/~jjg/8247760/webrev.00/