[gcc r12-4959] Fortran: Add more documentation for mixed-language programming [PR35276]

[Sandra Loosemore <sandra@gcc.gnu.org>]

https://gcc.gnu.org/g:452a0afbace650d7c0f2811e2b47e62dcd18e1fd

commit r12-4959-g452a0afbace650d7c0f2811e2b47e62dcd18e1fd
Author: Sandra Loosemore <sandra@codesourcery.com>
Date:   Fri Nov 5 14:01:03 2021 -0700

    Fortran: Add more documentation for mixed-language programming [PR35276]
    
    2021-11-05  Sandra Loosemore  <sandra@codesourcery.com>
    
            PR fortran/35276
    
            gcc/fortran/
            * gfortran.texi (Mixed-Language Programming): Talk about C++,
            and how to link.

Diff:
---
 gcc/fortran/gfortran.texi | 22 +++++++++++++++++-----
 1 file changed, 17 insertions(+), 5 deletions(-)

diff --git a/gcc/fortran/gfortran.texi b/gcc/fortran/gfortran.texi
index f3a961e4c1b..326470964b0 100644
--- a/gcc/fortran/gfortran.texi
+++ b/gcc/fortran/gfortran.texi
@@ -2718,11 +2718,23 @@ reading from the position marked previously.
 * Naming and argument-passing conventions::
 @end menu
 
-This chapter is about mixed-language interoperability, but also applies
-if one links Fortran code compiled by different compilers.  In most cases,
-use of the C Binding features of the Fortran 2003 standard is sufficient,
-and their use is highly recommended.
-
+This chapter is about mixed-language interoperability, but also
+applies if you link Fortran code compiled by different compilers.  In
+most cases, use of the C Binding features of the Fortran 2003 and
+later standards is sufficient.
+
+For example, it is possible to mix Fortran code with C++ code as well
+as C, if you declare the interface functions as @code{extern "C"} on
+the C++ side and @code{BIND(C)} on the Fortran side, and follow the
+rules for interoperability with C.  Note that you cannot manipulate
+C++ class objects in Fortran or vice versa except as opaque pointers.
+
+You can use the @command{gfortran} command to link both Fortran and
+non-Fortran code into the same program, or you can use @command{gcc}
+or @command{g++} if you also add an explicit @option{-lgfortran} option
+to link with the Fortran library.  If your main program is written in
+C or some other language instead of Fortran, see
+@ref{Non-Fortran Main Program}, below.
 
 @node Interoperability with C
 @section Interoperability with C