RFR: Add guide section about platform-dependent nature of jextract

[Jorn Vernee]

The current guide is lacking a discussion about the platform dependent nature of jextract, leaving users to wonder if and when the bindings generated by jextract can be used on different platforms.

This PR adds a couple of paragraphs to the guide to explain this.

Note: I've also fixed a typo in the README.

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Commit messages:
 - use italic
 - phrasing
 - Add notes on platform-dependent nature of jextract

Changes: https://git.openjdk.org/jextract/pull/237/files
  Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=237&range=00
  Stats: 22 lines in 2 files changed: 21 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jextract/pull/237.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/237/head:pull/237

PR: https://git.openjdk.org/jextract/pull/237