manual.texi Use I/O consistently, not I/O sometimes and i/o sometimes.

manual.texi	Fix a bunch of menu/section titles that are
manual.texi	missing @code{} around command names.
manual.texi	Use question marks at the end of questions.
manual.texi	Added missing 3.23.16 change note about
manual.texi	SLAVE START/STOP.


Docs/manual.texi:
  Use I/O consistently, not I/O sometimes and i/o sometimes.