Put comment lines after header for io.ascii commented header writing

This is a continuation of #3222; I finished the comment writing code and switched the order of the header and commented lines for commented-header format.
@taldcroft, @hamogu - is this okay?
Cut out the `(e.g. ...)` parenthetical comment to make space for this sentence:
```
The comments are defined as a list of strings in the input table `meta['comments']` element.
```
(This assumes convergence on #3256).