[Patch, fortran] Document more environment variables in gfortran.texi

[FX Coudert]

> 2007-05-21  Tobias Burnus  <burnus@net-b.de>
>
> 	* gfortran.texi: Document the GFORTRAN_ERROR_DUMPCORE and
> 	  GFORTRAN_ERROR_BACKTRACE environment variables.

OK, and thanks for covering my tracks!

FX