RFR: 8288660: JavaDoc should be more helpful if it doesn't recognize a tag [v2]

[Pavel Rappo]

> This PR makes JavaDoc and DocLint produce more helpful diagnostic output when encounter an unknown tag.
> 
> Before:
> 
> 
> MyClass.java:4: warning: no main description
>  * @implSpite {@linkpain Object#hashCode}}
>    ^
> MyClass.java:4: error: unknown tag: implSpite
>  * @implSpite {@linkpain Object#hashCode}}
>    ^
> MyClass.java:4: error: unknown tag: linkpain
>  * @implSpite {@linkpain Object#hashCode}}
>               ^
> MyClass.java:5: error: unknown tag: danger
>  * @danger
>    ^
> 
> 
> After:
> 
> 
>  * @implSpite {@linkpain Object#hashCode}}
>    ^
> MyClass.java:4: error: unknown tag: implSpite; the most similar tags are: implSpec, implNote
>  * @implSpite {@linkpain Object#hashCode}}
>    ^
> Note: An unknown tag has been reported. Mistyped? Forgot to add a custom tag or register a taglet?
> MyClass.java:4: error: unknown tag: linkpain; the most similar tags are: linkplain
>  * @implSpite {@linkpain Object#hashCode}}
>               ^
> MyClass.java:5: error: unknown tag: danger
>  * @danger
>    ^
> 
> 
> As you can see, the output has changed in two ways. Firstly, the tags that are similar to the unknown tag might be suggested. Secondly, an auxiliary note to help troubleshoot the unknown tag is provided. That note is provided once, close to the first reported unknown tag. This is done to not clutter the output in case multiple tags are reported.
> 
> For details, see the actual change.

Pavel Rappo has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - Shorten messages and simplify diagnostics
 - Merge branch 'master' into 8288660
 - Initial commit

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/15494/files
  - new: https://git.openjdk.org/jdk/pull/15494/files/8b51f1ba..8d4724ae

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15494&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15494&range=00-01

  Stats: 4446 lines in 153 files changed: 3324 ins; 618 del; 504 mod
  Patch: https://git.openjdk.org/jdk/pull/15494.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15494/head:pull/15494

PR: https://git.openjdk.org/jdk/pull/15494