Clarify relevance of JavadocTest_1_3 ...

Some of `o.e.j.core.tests.compiler.regression.JavadocTest*` are no longer running during builds, because those are included in the test suite only when their compliance level is **matched exactly**, see `JavadocTest.suite()`. This differs from other suites where only a minimum compliance is required, not an exact match.

Locally running, e.g., `JavadocTest_1_3` currently shows lots of failures. Likely, https://github.com/eclipse-jdt/eclipse.jdt.core/discussions/820 will tell us that we don't need to test 1.3 comliance any more, and at first look, some tests indeed only check if javadoc is rejected that is legal only since 1.5, but I guess there may also be a bunch of tests, that are valid *since 1.3* without being specific to that version.

Start with JavadocTest.suite() and inspect the JavadocTest* cases, especially JavadocTest_1_3, which can be run locally and currently reports failures. Read discussion 820 for the compliance context, then determine which tests are still relevant and what suite behavior should be retained; done means the relevant tests run under the intended compliance rules and pass.

Written by the indexing model from the issue text.