RFR: 8247957: remove doclint support for HTML 4 [v3]

[Yoshiki Sato]

On Thu, 17 Dec 2020 04:48:44 GMT, Jonathan Gibbons <jjg at openjdk.org> wrote:

>> OK: valid
>> OBSOLETE: obsolete, deprecated, but still supported (valid)
>> UNSUPPORTED: ever supported but no longer supported (invalid)
>> INVALID: the rest of others (invalid)
>> 
>> UNSUPPORTED can be used if we would like to choose a friendly message instead of saying "unknown tag" only.
>> OBSOLETE is not used anywhere in this commit.  Although HTML5 has some obsolete features, [JDK-8215577](https://bugs.openjdk.java.net/browse/JDK-8215577) didn't define them as valid features if my understanding is correct.  So I chose not to allow obsolete features in order to avoid inconsistency.
>
> For both `ElemKind` and `AttrKind` there only seem to be two kinds:
> * valid
> * previously valid
> 
> For these two cases, `OK` is obviously reasonable for `valid`, but `OBSOLETE` seems a better fit than `UNSUPPORTED`, but you could also use `HTML4` or `OLD_HTML4` or something like that to indicate why we're keeping the name around for better messages.  Or, stay with `UNSUPPORTED` but add a doc comment explaining that it was previously supported but no longer supported

Ok, I will use `HTML4`.

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PR: https://git.openjdk.java.net/jdk/pull/893