RFR: JDK-8300914: Allow `@` as an escape in documentation comments [v6]

[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 with a new target base due to a merge or a rebase. The pull request now contains seven commits:

 - post-merge fix
 - Merge remote-tracking branch 'upstream/master' into 8300914.at-escapes
 - Improve spec wording
 - Merge with upstream/master
 - Address review feedback
 - Merge with upstream/master
 - JDK-8301294: Allow `@` as an escape in documentation comments

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Changes: https://git.openjdk.org/jdk/pull/12372/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12372&range=05
  Stats: 456 lines in 16 files changed: 436 ins; 0 del; 20 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