Polished Ravenwood docs.

* Highlight temporary strategies for JNI.
* Clarify current `TEST_MAPPING` status.
* Mention `@IgnoreUnderRavenwood` support at class level.
* Mention strategy for bivalent tests using resources.
* Concrete example of temporary directory.

Bug: 292141694
Test: atest CtsUtilTestCasesRavenwood CtsUtilTestCases
Change-Id: If0903296697049d248c50ab46b01b273aab26c9f