RFR: 7903257: jextract should generate javadoc comments

[Athijegannathan Sundararajan]

Generating C signature as doc comments.

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Commit messages:
 - 7903257: jextract should generate javadoc comments

Changes: https://git.openjdk.org/jextract/pull/85/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=85&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903257
  Stats: 582 lines in 16 files changed: 560 ins; 3 del; 19 mod
  Patch: https://git.openjdk.org/jextract/pull/85.diff
  Fetch: git fetch https://git.openjdk.org/jextract pull/85/head:pull/85

PR: https://git.openjdk.org/jextract/pull/85