Fortran: Fix that/which usage in the manual.

In English usage, "that" introduces a restrictive clause while "which"
introduces a non-restrictive or descriptive clause.  "That" is almost
never preceded by a comma while "which" often is.  The Fortran manual
had many instances where these uses were reversed, or where a comma
was used with "that"; this patch fixes them.  In some cases I have
substituted less convoluted wording instead.

gcc/fortran/ChangeLog
	* gfortran.texi: Clean up that/which usage throughout the file.
	* intrinsic.texi: Likewise.
	* invoke.texi: Likewise.