RFR: Minor tweaks in jextract codegen

[Maurizio Cimadamore]

When running some examples, I noted that:

* jextract is not adding javadoc to the function wrappers in the header class (instead the javadoc is added to the MH getter)
* the new `reinterpret` method still use the parameter name `scope` instead of `arena`

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/jextract/pull/183/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=183&range=00
  Stats: 10 lines in 2 files changed: 4 ins; 1 del; 5 mod
  Patch: https://git.openjdk.org/jextract/pull/183.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/183/head:pull/183

PR: https://git.openjdk.org/jextract/pull/183