Fortran: Fixes for markup, typos, and indexing in manual

Fortran: Fixes for markup, typos, and indexing in manual

While working on something else I noticed there were numerous places
in the GNU Fortran manual with incorrect/missing Texinfo markup.  I
made a pass through about the first third of the manual (not yet the
coarray API or intrinsics documentation) to fix at least some of those
issues, plus some typos and missing @cindex entries.

There shouldn't be any semantic changes to the documentation in this patch.

gcc/fortran/ChangeLog
	* gfortran.texi: Fix markup, typos, and indexing throughout the
	file.
	* invoke.texi: Likewise.