RFR: JDK-8253733: Cleanup internal taglet API

[Pavel Rappo]

On Tue, 29 Sep 2020 16:42:09 GMT, Jonathan Gibbons <jjg at openjdk.org> wrote:

> This is a general cleanup of various classes related to the handling of tags and taglets in the standard doclet.
> 
> The initial motivation was to clean up static methods on `TagletWriter` that took a `TagletWriter` as a parameter.
> These methods are converted into instance methods, and the call sites simplified as well.
> Another area of cleanup is to make clear which methods are used for inline tags and which are used for block tags.
> 
> Some dead code is removed, and the doc comments are generally cleaned up as well.  More details are given in the JBS
> issue.

These classes might need a copyright year update:

1. `jdk.javadoc.internal.doclets.toolkit.taglets.ValueTaglet`
2. `jdk.javadoc.internal.doclets.toolkit.taglets.SystemPropertyTaglet`
3. `jdk.javadoc.internal.doclets.toolkit.taglets.SummaryTaglet`
4. `jdk.javadoc.internal.doclets.toolkit.taglets.SeeTaglet`
5. `jdk.javadoc.internal.doclets.toolkit.taglets.ReturnTaglet`
6. `jdk.javadoc.internal.doclets.toolkit.taglets.LiteralTaglet`
7. `jdk.javadoc.internal.doclets.toolkit.taglets.IndexTaglet`
8. `jdk.javadoc.internal.doclets.toolkit.taglets.DocRootTaglet`
9. `jdk.javadoc.internal.doclets.toolkit.taglets.DeprecatedTaglet`
10. `jdk.javadoc.internal.doclets.toolkit.taglets.CodeTaglet`

It's easier to update copyright years unconditionally than to think whether or not a change amounts to a "substantial
change".

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PR: https://git.openjdk.java.net/jdk/pull/406