Add documentation for WildflyExternalizedToOperatorInfinispanSessionExpirationIT.java

[fabiobrz]

Recently a new test was added, i.e. https://github.com/Intersmash/intersmash-tests/blob/main/testsuite/src/test/java/org/jboss/intersmash/tests/wildfly/web/cache/offload/infinispan/WildflyExternalizedToOperatorInfinispanSessionExpirationIT.java but it is not documented, neither in the the repository README.md, nor in the https://github.com/Intersmash/intersmash-tests/tree/main/testsuite#readme.

We should add documentation for this new test, and evaluate whether to move it to a separate same-level package (e.g.: externalize?).