Update FailOnSpecificWarningsPlugin doc to explain usage vs verifyWarningFree

[timjohns-mw]

Need to explain when you would use FailOnSpecificWarningsPlugin vs verifyWarningFree. Explanation is that:

1. the plugin applies across the entire test suite whereas verifyWarningFree is for use within a test point;
2. verifyWarningFree will produce a qualification failure if any warning is produced (like the built-in FailOnWarningsPlugin) whereas FailOnSpecificWarningsPlugin will only produce a qualification failure if specific warnings are produced.

verifyWarningNotProduced might be a useful addition to this toolbox.