RFR: 8286773: cleanup @returns in sun.security classes

[John Jiang]

The below sun.security classes should use `@return` rather than `@returns`.
sun/security/tools/keytool/Main.java
sun/security/util/DerValue.java

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Commit messages:
 - 8286773: cleanup @returns in sun.security classes

Changes: https://git.openjdk.java.net/jdk/pull/8714/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8714&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286773
  Stats: 4 lines in 2 files changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8714.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8714/head:pull/8714

PR: https://git.openjdk.java.net/jdk/pull/8714