RFR: 8288660: JavaDoc should be more helpful if it doesn't recognize a tag

[Pavel Rappo]

This PR makes JavaDoc and DocLint produce more helpful diagnostic output when encounter an unknown tag.

Before:


MyClass.java:4: warning: no main description
 * @implSpite {@linkpain Object#hashCode}}
   ^
MyClass.java:4: error: unknown tag: implSpite
 * @implSpite {@linkpain Object#hashCode}}
   ^
MyClass.java:4: error: unknown tag: linkpain
 * @implSpite {@linkpain Object#hashCode}}
              ^
MyClass.java:5: error: unknown tag: danger
 * @danger
   ^


After:


 * @implSpite {@linkpain Object#hashCode}}
   ^
MyClass.java:4: error: unknown tag: implSpite; the most similar tags are: implSpec, implNote
 * @implSpite {@linkpain Object#hashCode}}
   ^
Note: An unknown tag has been reported. Mistyped? Forgot to add a custom tag or register a taglet?
MyClass.java:4: error: unknown tag: linkpain; the most similar tags are: linkplain
 * @implSpite {@linkpain Object#hashCode}}
              ^
MyClass.java:5: error: unknown tag: danger
 * @danger
   ^


As you can see, the output has changed in two ways. Firstly, the tags that are similar to the unknown tag might be suggested. Secondly, an auxiliary note to help troubleshoot the unknown tag is provided. That note is provided once, close to the first reported unknown tag. This is done to not clutter the output in case multiple tags are reported.

For details, see the actual change.

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

Commit messages:
 - Initial commit

Changes: https://git.openjdk.org/jdk/pull/15494/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15494&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8288660
  Stats: 238 lines in 11 files changed: 214 ins; 10 del; 14 mod
  Patch: https://git.openjdk.org/jdk/pull/15494.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15494/head:pull/15494

PR: https://git.openjdk.org/jdk/pull/15494