RFR: JDK-8300914: Allow `@` as an escape in documentation comments [v2]

[Jonathan Gibbons]

On Fri, 3 Feb 2023 00:25:22 GMT, Jonathan Gibbons <jjg at openjdk.org> wrote:

>> '/' cannot be part of a tag name.
>> 
>> A tag name starts with a character for which `isUnicodeIdentifierStart` is true, followed by zero or more characters for which `isUnicodeIdentifierPart` is true or is one of `.`, `-`, `:`.  See `DocCommentParser.readTagName` for verification. (We should update the Doc Comment Spec for that as well.)
>> 
>> So, the text of error messages above are correct, even if the positions require tweaking.
>
> Note that all the proposed escape sequences are context-sensitive, and are only enabled in appropriate circumstances.

updated code, comments and spec.

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