RFR: JDK-8301294: Allow `@` as an escape in documentation comments

[Jonathan Gibbons]

Please review a moderately simple update to permit the use of `@` as the escape character in a limited set of escape sequences.

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Commit messages:
 - JDK-8301294: Allow `@` as an escape in documentation comments

Changes: https://git.openjdk.org/jdk/pull/12372/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12372&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301294
  Stats: 384 lines in 18 files changed: 351 ins; 6 del; 27 mod
  Patch: https://git.openjdk.org/jdk/pull/12372.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12372/head:pull/12372

PR: https://git.openjdk.org/jdk/pull/12372