RFR: 8267995: Add reference to JVMS class file format in Lookup::defineHiddenClass

[Mandy Chung]

Trivial javadoc change to add a reference to JVMS 4.1 in `Lookup::defineHiddenClass` such that the reference like `this_class` would be referred to JVMS for details.

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Commit messages:
 - 8267995: Add reference to JVMS class file format in Lookup::defineHiddenClass

Changes: https://git.openjdk.java.net/jdk/pull/4340/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4340&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8267995
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4340.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4340/head:pull/4340

PR: https://git.openjdk.java.net/jdk/pull/4340