* gfortran.texi: Added TeX support for document parts;

rearranged existing text into "About GNU Fortran",
"Invoking GNU Fortran", and "Language Reference" parts.



git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@119981 138bc75d-0d04-0410-961f-82ee72b054a4