[patch, Fortran, doc] Mention generating C prototypes from Fortran

[Thomas Koenig]

Hello world,

I noticed that there is no mention of automatically generating
C prototypes and declarations in the relevant section of
the docs, "Interoperability with C". This patch remedies that.

OK for trunk?

Best regards

        Thomas

gcc/fortran/ChangeLog:

        * gfortran.texi: New subsection "Generating C prototypes from
        Fortran".