Fortran: Fix hyphenation errors in the manual

Fortran: Fix hyphenation errors in the manual

When looking through the gfortran manual, I noted some problems with
hyphens being used where they're not correct or necessary,
e.g. "non-standard" vs "nonstandard", "null-pointer" vs "null pointer"
(as a noun), etc.  I've made a pass through the documentation to
correct at least some of those uses.

gcc/fortran/ChangeLog
	* gfortran.texi: Get rid of some unnecessary hyphens throughout
	the file.
	* invoke.texi: Likewise.