[fpc-pascal] Docs: portability differences between Borland/FPC

[Marco van de Voort]

In our previous episode, Martok said:
> There is the old "Porting TP" document at
> <https://www.freepascal.org/port.var>, but it is rather incomplete (and
> probably on an older language level).  It also covers mostly syntactic
> differences - things a programmer will notice because of the error
> messages.

Summary: behaviour with range checks off is implementation defined?