Stop makeinfo 5.1 from erroring out with "@itemx must follow @item".

These were the errors.

 ./vectors.texi:292: @itemx must follow @item
 ./vectors.texi:302: @itemx must follow @item
 ./strings.texi:360: @itemx must follow @item
 ./bytevectors.texi:807: @itemx must follow @item

The cause was that this is legal:

 @item foo
 @itemx bar

while this is illegal:

 @item foo

 @itemx bar

as is this:

 @item foo
 text
 @itemx bar
 more text

and this:

 @item foo
 text

 @itemx bar
 more text