RFR: 8346659: SnippetTaglet should report an error if provided ambiguous links [v9]

[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 two additional commits since the last revision:

 - Revert "Improve error message by pointing to location of links"
   
   This reverts commit 9b2107d90203febd9615e7622816432751c3f9e2.
 - Revert "trim error message"
   
   This reverts commit e0d70a410a1149efa07f6486334fdcad3e31a3c9.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/23328/files
  - new: https://git.openjdk.org/jdk/pull/23328/files/e0d70a41..a808e2c8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23328&range=08
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23328&range=07-08

  Stats: 34 lines in 3 files changed: 0 ins; 15 del; 19 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