RFR: Add jextract guide [v6]

[Jorn Vernee]

On Mon, 15 Apr 2024 14:11:13 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> `only` is relevant here. If a macro is defined using `#define` _and_ on the command line, jextract will generate an accessor.
>
> I understand what you wanted to stress - but IMHO `only` is still redundant. E.g. "when a macro is defined using -D" is still plenty clear? Using `only` seems to convey some kind of deeper distinction e.g. that macros with -D are somehow "less macros" than the other ones. At least it's how it reads to me.

Ok, removed.

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PR Review Comment: https://git.openjdk.org/jextract/pull/231#discussion_r1566521615