Fortran: Tidy subheadings in Fortran documentation [PR47928]

This is a preparatory patch for the main documentation changes requested
in the issue.

gcc/fortran/ChangeLog
	PR fortran/47928
	* gfortran.texi: Consistently use "@emph{Notes}:" instead of
	other spellings.
	* intrinsic.texi: Likewise.  Also fix an inconsistent capitalization
	and remove a redundant "Standard" entry.