RFR: 8276889: Improve compatibility discussion in instanceKlass.cpp

[Coleen Phillimore]

I removed the confusing comment that was missing some words, and linked the RFE to this one that the comment was referring to.  At one point in time, I really wanted to remove this code (still do but not as much now).  With JVMTI Heap functions deprecated JDK-8268242, maybe soon.
Please review this trivial change.

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Commit messages:
 - 8276889: Improve compatibility discussion in instanceKlass.cpp

Changes: https://git.openjdk.java.net/jdk/pull/6340/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6340&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276889
  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6340.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6340/head:pull/6340

PR: https://git.openjdk.java.net/jdk/pull/6340