#1232 Improve javadoc about OperandStackDetector

[Kengo TODA]

I implemented some detectors which extends OperandStackDetector, and I felt that it is better to improve javadoc especially about OpcodeStack.CustomUserValue annotation. I didn't know this annotation and cost some time to find why my detector had problem.

Please use attached patch to master branch, or fetch my branch from my repository at GitHub.

[William Pugh]

thanks. Fixed