RFR: 8346659: SnippetTaglet should report an error if provided ambiguous links [v8]

[Nizar Benalla]

> Some javadoc snippets can match multiple links to the same content, leading to different results in different javadoc runs.
> This patch proposes emitting an error when such cases are encountered.
> 
> There is a very trivial, unrelated change in `TestGlobalHtml.java` because I noticed some whitespace wasn't right.

Nizar Benalla has updated the pull request incrementally with one additional commit since the last revision:

  trim error message

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/23328/files
  - new: https://git.openjdk.org/jdk/pull/23328/files/9b2107d9..e0d70a41

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23328&range=07
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23328&range=06-07

  Stats: 6 lines in 1 file changed: 0 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/23328.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23328/head:pull/23328

PR: https://git.openjdk.org/jdk/pull/23328