RFR: 8279189: Inaccurate comment about class VMThread

[Xin Liu]

The comment is inaccurate now. VMThread is different from the primordial thread. I guess the comment is based on the old codebase.
Because VMThread is singleton, I put the Constructor and Destructor in "private" section. clients use static member functions create/destroy.

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Commit messages:
 - 8279189: Inaccurate comment about class VMThread

Changes: https://git.openjdk.java.net/jdk/pull/6922/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6922&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279189
  Stats: 4 lines in 1 file changed: 1 ins; 2 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6922.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6922/head:pull/6922

PR: https://git.openjdk.java.net/jdk/pull/6922