RFR: 8261650: Add a comment with details for MTLVC_MAX_INDEX

[Ajit Ghaisas]

This is a trivial fix which adds more details about the constant MTLVC_MAX_INDEX.
This had come up during the review of [JDK-8261632](https://bugs.openjdk.java.net/browse/JDK-8261632).

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Commit messages:
 - Add details about MTLVC_MAX_INDEX

Changes: https://git.openjdk.java.net/jdk/pull/8592/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8592&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261650
  Stats: 8 lines in 1 file changed: 7 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8592.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8592/head:pull/8592

PR: https://git.openjdk.java.net/jdk/pull/8592