Fortran manual: Update section on Interoperability with C

Fortran manual: Update section on Interoperability with C

2021-11-01  Sandra Loosemore  <sandra@codesourcery.com>

	gcc/fortran/
	* gfortran.texi (Interoperability with C): Copy-editing.  Add
	more index entries.
	(Intrinsic Types): Likewise.
	(Derived Types and struct): Likewise.
	(Interoperable Global Variables): Likewise.
	(Interoperable Subroutines and Functions): Likewise.
	(Working with C Pointers): Likewise.
	(Further Interoperability of Fortran with C): Likewise.  Rewrite
	to reflect that this is now fully supported by gfortran.