RFR: 7903639: Add javadoc to all jextract generated API points

[Maurizio Cimadamore]

This PR adds simple javadoc text to all the helper functions generated by jextract:
* struct allocation methods
* struct slicing methods
* struct reinterpret methods
* function pointer allocation method
* function pointer invoke method

Following some offline discussions, I've decided to keep the comments very dry and simple (to avoid copying reams and reams of text into multiple classes). Also, the javadoc does not contain any `@param` or `@return` tag.

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/jextract/pull/193/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=193&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903639
  Stats: 63 lines in 4 files changed: 48 ins; 2 del; 13 mod
  Patch: https://git.openjdk.org/jextract/pull/193.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/193/head:pull/193

PR: https://git.openjdk.org/jextract/pull/193