We named the argument `position` in the code and doc arguments, but the
rest of the documentation referred to `index` instead. That was maybe
meant to hint at the child property named :index, but we can simply be
fully clear here. We can call the argument `index_`, replacing the local
variable with that name, thus avoiding any possible confusion with the
unrelated ::get-child-position, and refer users to :index for completion

`index_` is used instead of plain `index` in case anyone is #including
<strings.h> and getting the old index() function superseded by strchr();
see GNOME/gtk!932 (comment 531149)