RFR: 8314448: Coordinate DocLint and JavaDoc to report on unknown tags

[Pavel Rappo]

Please review this bug fix for reporting on unknown javadoc tags.

Aside from a somewhat irrelevant test for DocLint[^1], there are no direct tests for reporting on unknown javadoc tags. That reporting, as it turns out, is ridden with bugs. This PR fixes quite a few of those bugs and adds missing tests.

Note, that a test for miscased tags (i.e. "{0} is an unknown tag -- same as a known tag except for case") is not added, as the matching mechanism is to be superseded by a more general one, based on a string distance function, in [JDK-8288660](https://bugs.openjdk.org/browse/JDK-8288660).

Also note, that while javadoc emits a warning on an unknown tag, DocLint emits an error. I haven't changed warning to error in javadoc for (better) backward compatibility. This way, an unknown tag won't fail javadoc run (unless of course `-Werror` is specified). That said, in the future this decision may be revised.

[^1]: test/langtools/tools/doclint/CustomTagTest.java

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Commit messages:
 - fix: add tests
 - fix: improve `assert` in checkUnknownTag
 - fix: coordinate DocLint with JavaDoc
 - fix: do not stop on first disabled tag
 - refactor: rearrange comments
 - refactor: remove redundant check
 - fix: check unknown tags
 - refactor: delete dead code
 - refactor: declare a variable closer to use
 - refactor: share `continue`

Changes: https://git.openjdk.org/jdk/pull/15345/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15345&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314448
  Stats: 194 lines in 4 files changed: 126 ins; 18 del; 50 mod
  Patch: https://git.openjdk.org/jdk/pull/15345.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15345/head:pull/15345

PR: https://git.openjdk.org/jdk/pull/15345