[jdk19] RFR: 6509045: {@inheritDoc} only copies one instance of the specified exception

[Pavel Rappo]

This is the second of the two PRs that fix inheritance for multiple `@throws` tags for a particular exception.

The first PR (8067757, #95) fixed inheritance for multiple `@throws` tags for an exception referred to by a `throws` clause. This PR fixes inheritance for multiple `@throws` tags for an exception referred to by an `@throws` tag.

It is expected that if an overriding method has an `@throws` tag that has an `{@inheritDoc}` in its description, then the documentation will inherit all tags that describe that same exception in the overridden methods. For example, support the setup looks like this:

* Overrider
   ```
   @throws NullPointerException if this
   @throws NullPointerException if that
   ```

* Overridee
   ```
   @throws NullPointerException {@inheritDoc}
   ```
Then the overridee's documentation should look like this:

Throws:
NullPointerException - if this
NullPointerException - if that 


While logical and expected, such behavior is different from other use cases of `{@inheritDoc}`. In other use cases `{@inheritDoc}` expands locally: it replaces itself with a list of tree nodes. Such a replacement doesn't affect any tree node other than its parent.

`{@inheritDoc}` expands locally everywhere, such as in the main description, `@param`, and `@return` tags, but it is not enough for `@throws`, which sometimes needs to amend the **grandparent** node to replace a `@throws` node with multiple `@throws` nodes. Amending this behavior globally just for `@throws` would be impractical.

For now, it seems reasonable to fix this by overriding the `{@inheritDoc}` processing just for `@throws`. This PR gives `ThrowsTaglet` an ability to peek at `@throws` description to see if there are any `{@inheritDoc}` there.

There's one limitation though. Suppose `@throws` documentation is both amended and inhertied from multiple `@throws` like this:

* Overridder

@throws NullPointerException if this
@throws NullPointerException if that


* Overridee

@throws NullPointerException "{@inheritDoc}"


It's not clear what to do in such a case. If only one of either amendment (a `"` quote before and after) or multiple-inheritance is present, it's clear what to do. But not when both are present. For now, such a situation is detected and flagged as an error (a test is provided).

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

Commit messages:
 - fix: raise an error not warning
 - refactor: improve the `expand` method
 - fix: add a combined test
 - refactor: delete unused imports
 - refactor: further split throwsTagsOutput
 - refactor: improve extracted method (3)
 - refactor: improve extracted method (2)
 - refactor: improve extracted method
 - fix: extract a method
 - fix: use the element tags were inherited from
 - ... and 1 more: https://git.openjdk.org/jdk19/compare/0526402a...702ff132

Changes: https://git.openjdk.org/jdk19/pull/116/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk19&pr=116&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-6509045
  Stats: 271 lines in 2 files changed: 261 ins; 2 del; 8 mod
  Patch: https://git.openjdk.org/jdk19/pull/116.diff
  Fetch: git fetch https://git.openjdk.org/jdk19 pull/116/head:pull/116

PR: https://git.openjdk.org/jdk19/pull/116