RFR: JDK-8325266: Enable this-escape javac warning in jdk.javadoc

[Joe Darcy]

The jdk.javadoc module doesn't need the this-escape warning disabled to build; it should be enabled.

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Commit messages:
 - JDK-8325266: Enable this-escape javac warning in jdk.javadoc

Changes: https://git.openjdk.org/jdk/pull/17715/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17715&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325266
  Stats: 3 lines in 1 file changed: 0 ins; 2 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/17715.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17715/head:pull/17715

PR: https://git.openjdk.org/jdk/pull/17715