RFR: JDK-8300914: Allow `@` as an escape in documentation comments [v3]

[Jonathan Gibbons]

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

Jonathan Gibbons has updated the pull request incrementally with one additional commit since the last revision:

  Address review feedback

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12372/files
  - new: https://git.openjdk.org/jdk/pull/12372/files/cf207d54..f54ddcac

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12372&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12372&range=01-02

  Stats: 95 lines in 6 files changed: 85 ins; 5 del; 5 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