[patch, fortran, docs] Document sequential unformatted file structure

[Jerry DeLisle]

On 08/12/2017 05:16 AM, Thomas Koenig wrote:
> Hello world,
> 
> seems like we never got around to documenting the structure of
> our unformatted sequential files.  Well, here is a patch for that.
> 
> Regards
> 
>     Thomas
> 
> 2017-08-12  Thomas Koenig  <tkoenig@gcc.gnu.org>
> 
>         * gfortran.texi: Document format of unformatted sequential files.

+with the @option{-fmax-subrecord-length} option, altough this is
+rarely useful. If the lenght of a logical record exceeds this limit,
  spelling -------------^^^

+the data is distributed among several subrecords.
+

Otherwise, OK

Jerry