RFR: JDK-8273194: Document the two possible cases when Lookup::ensureInitialized returns

[Mandy Chung]

Improve the specification to document the cases when `Lookup::ensureInitialized` returns as specified JVMS 5.5 and matches the implementation.

Please also review CSR: https://bugs.openjdk.java.net/browse/JDK-8273253

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Commit messages:
 - JDK-8273194: Document the two possible cases when Lookup::ensureInitialized returns

Changes: https://git.openjdk.java.net/jdk/pull/5343/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5343&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273194
  Stats: 7 lines in 1 file changed: 6 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5343.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5343/head:pull/5343

PR: https://git.openjdk.java.net/jdk/pull/5343