RFR: JDK-8252937: Correct "no comment" warnings from javadoc in java.math …

[Brian Burkhalter]

Please review this simple change to clean up some warnings encountered when javadoc is run.

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Commit messages:
 - JDK-8252937: Correct "no comment" warnings from javadoc in java.math package

Changes: https://git.openjdk.java.net/jdk/pull/415/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=415&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252937
  Stats: 16 lines in 3 files changed: 12 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/415.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/415/head:pull/415

PR: https://git.openjdk.java.net/jdk/pull/415