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

[Jonathan Gibbons]

On Thu, 12 Nov 2020 03:10:01 GMT, Yoshiki Sato <ysatowse at openjdk.org> wrote:

>> src/jdk.javadoc/share/classes/jdk/javadoc/internal/doclint/HtmlTag.java line 410:
>> 
>>> 408:         OBSOLETE,
>>> 409:         UNSUPPORTED
>>> 410:     }
>> 
>> On one hand, I don't think we need this level of detail, but on the other, I see it closely matches `AttrKind`, so OK.
>> 
>> Is there are useful distinction between INVALID / OBSOLETE / UNSUPPORTED ?
>
> 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

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