RFR: 8256917: Use combo @returns tag in java.compiler javadoc

[Joe Darcy]

Take advantage of upgraded dual-use @return tag.

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Commit messages:
 - Take advantage of dual-use @return tag.

Changes: https://git.openjdk.java.net/jdk/pull/1708/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1708&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256917
  Stats: 331 lines in 33 files changed: 2 ins; 170 del; 159 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1708.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1708/head:pull/1708

PR: https://git.openjdk.java.net/jdk/pull/1708