RFR: Add jextract guide [v5]

[Jorn Vernee]

On Sat, 13 Apr 2024 05:36:09 GMT, Nir Lisker <nlisker at openjdk.org> wrote:

> > > Did you leave out "Bitfields" on purpose?
> > 
> > 
> > Jextract does not support bitfields (due to deficiencies in the libclang API). That's why there's no section for them :-)
> 
> Then maybe mention that :)

I've added a section about unsupported features. (also to explain some of the warnings a user may see)

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

PR Comment: https://git.openjdk.org/jextract/pull/231#issuecomment-2056835517