Update FailOnSpecificWarningsPlugin doc to explain usage vs verifyWarningFree

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.

Locate the documentation for FailOnSpecificWarningsPlugin and verifyWarningFree, then compare how each is used and how warnings cause qualification failures. Update the usage guidance to explain their scope and warning behavior, and decide whether verifyWarningNotProduced belongs in the toolbox documentation.