RFR: 8285488: Improve DocFinder

[Pavel Rappo]

Please have a look at this work-in-progress PR. The reason this is a (normal) PR rather than a more suitable draft PR is to mainly trigger a discussion on the mailing list on whether the suggested approach to solve multiple intertwined issues of exception documentation inheritance is a correct one.

In a nutshell, it turns out that the combination of `{@throws}` and `{@inheritDoc}` is quite complex. It's more complex than a combination of `{@inheritDoc}` with any other tag or `{@inheritDoc}` on its own. To account for that complexity, handling of `{@inheritDoc}` in `{@throws}` is lifted to `ThrowsTaglet`.

The said complexity stems from two facts:

1. Processing of `{@inheritDoc}` is context free and is done by replacing `{@inheritDoc}` with the tags it expands to. That model does not account for `@throws` where `{@inheritDoc}` sometimes expands to multiple `@throws` tags, which correspond to _separate_ entries in the "Throws:" section of a method detail. Read: we change the exception section, not a description of one of its entries.

2. Corresponding exception types in the hierarchy (i.e. matching `{@inheritDoc}` with exception documentation it must inherit) is not always clear-cut. This becomes especially apparent when type variables are involved.

History
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The work started as an attempt to fix JDK-8285488, hence the issue number for the PR. However, along its course the PR solved other issues, which will be soon added to the PR:

* 8287796: Stop auto-inheriting documentation for subclasses of exceptions whose documentation is inherited
* 8291869: Match exceptions using types of `javax.lang.model`, not strings
* 8285277: Expand `{@inheritDoc}` in `@throws` fully
* 8288045: Clean up ParamTaglet
* 8288046: Clean up ThrowsTaglet

As of today
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* All tests (both existing and newly introduced) pass.
* JDK API Documentation is the same, except for two files. In the first file, `docs/api/java.management.rmi/javax/management/remote/rmi/RMIConnectionImpl_Stub.html`, the order of exceptions has changed, which is insignificant. As for the second file, `docs/api/java.management/javax/management/MBeanServer.html`, the new warning is generated and erroneous input added to the corresponding page. The issue has to be addressed on the component side and is tracked by JDK-8295151.

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Commit messages:
 - fix: process @throws {@inheritDoc} in ThrowsTaglet
 - Merge branch 'master' into 8285488
 - unrelated: use Text, not RawHtml for exceptions
 - fix: remove auto-inheriting of subexceptions
 - Merge branch 'master' into 8285488
 - Merge branch 'master' into 8285488
 - refactor: simplify compound ops on set
 - refactor: abstract out "Throws:" section building
 - refactor: support exceptions in DocFinder.search
 - refactor: unused declaration
 - ... and 29 more: https://git.openjdk.org/jdk/compare/172006c0...10cfc7f9

Changes: https://git.openjdk.org/jdk/pull/10746/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10746&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8285488
  Stats: 2020 lines in 25 files changed: 1407 ins; 322 del; 291 mod
  Patch: https://git.openjdk.org/jdk/pull/10746.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10746/head:pull/10746

PR: https://git.openjdk.org/jdk/pull/10746