RFR: 8264606: More comment for ECDH public key validation

[Xue-Lei Andrew Fan]

Improve the readability by adding more comments in the ECDH public key validation implementation in JDK.

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Commit messages:
 - 8264606: More comment for ECDH public key validation

Changes: https://git.openjdk.java.net/jdk/pull/3313/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3313&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264606
  Stats: 25 lines in 1 file changed: 18 ins; 1 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3313.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3313/head:pull/3313

PR: https://git.openjdk.java.net/jdk/pull/3313