fix javadoc issues

[greenwoodma]

With Java 8 javadoc issues now cause a build failure unless we suppress the issues. We suppress the issues because there are so many of them in the existing codebase. Eventually we should stop suppressing the errors and actually fix them as this will make the javadoc better as well.

This issue is listed here, even though it really applies to all plugins because the errors are being suppressed by a configuration in the gate-plugin-base pom.xml