RFR: Add guide section about platform-dependent nature of jextract [v2]

[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.

Jorn Vernee has updated the pull request incrementally with one additional commit since the last revision:

  Apply suggestions from code review
  
  Co-authored-by: Maurizio Cimadamore <54672762+mcimadamore at users.noreply.github.com>

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

Changes:
  - all: https://git.openjdk.org/jextract/pull/237/files
  - new: https://git.openjdk.org/jextract/pull/237/files/aaddfb65..a5be8cbf

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jextract&pr=237&range=01
 - incr: https://webrevs.openjdk.org/?repo=jextract&pr=237&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 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