RFR: 8340404: Clarify CharsetProvider deployment on application module path [v6]

[Justin Lu]

> Please review this PR and [CSR](https://bugs.openjdk.org/browse/JDK-8340412) which clarifies that a CharsetProvider SPI can be provided via the application module path.
> 
> This came as a result from a comment on the PR of JDK-8339735. 
> 
> I included some minor wording on deploying as a module, with a link to `Service Loader's` _Developing Service Providers section_ to handle the heavy lifting. However, I am happy to adjust as needed, if it is felt that more info should be laid out directly on `CharsetProvider's` class description itself.
> 
> An associated test is provided to confirm that a `CharsetProvider` _can_ be deployed as a module.

Justin Lu has updated the pull request incrementally with one additional commit since the last revision:

  review: spec correction - system class loader used, not context

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/21076/files
  - new: https://git.openjdk.org/jdk/pull/21076/files/c4056ba8..d90f6ca0

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21076&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21076&range=04-05

  Stats: 6 lines in 1 file changed: 0 ins; 1 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/21076.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21076/head:pull/21076

PR: https://git.openjdk.org/jdk/pull/21076