RFR: 8274528: Add comment to explain an HKDF optimization in SSLSecretDerivation

[Bradford Wetmore]

See comment, + a couple minor cleanups.

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Commit messages:
 - Minor cleanup.

Changes: https://git.openjdk.java.net/jdk/pull/5765/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5765&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274528
  Stats: 13 lines in 1 file changed: 10 ins; 1 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5765.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5765/head:pull/5765

PR: https://git.openjdk.java.net/jdk/pull/5765