[PATCH, Fortran] PING: Extend gfc_code internal documentation

[Dominique Dhumieres]

Motivated by the Tobias' comments and from another non-native speaker:

> +this, were the horizontal link-chains are those induced by the @code{next}
         ^^^^
         where

> Instead of "being", I'd use "which is".

I have been told by a colleague that American kids are doing "which hunt"
(replacing "which" by "that"). Altough I am pretty bad at this game, I'ld
use "that is" here.

Otherwise I think it's a great idea to document the internal guts of gfortran.

Dominique