RFR: 8263978: Clarify why 0 argument is ignored in SecureRandom::setSeed

[Weijun Wang]

We don't use `digest` anymore but still need to ignore the 0 argument.

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Commit messages:
 - 8263978: Clarify why 0 argument is ignored in SecureRandom::setSeed

Changes: https://git.openjdk.java.net/jdk/pull/3124/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3124&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263978
  Stats: 5 lines in 1 file changed: 3 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3124.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3124/head:pull/3124

PR: https://git.openjdk.java.net/jdk/pull/3124