[later iteration] Explain X of XX numbers outside of Level

[shawna-slh]

Ref: #168

I think we should explain it. Probably at the bottom of the Overview page, collapsed. Maybe under a new heading at the end. Or, maybe combine "Structure of this Tool" and "Result Choices" and this into something like:

More about using this ATAG Report Tool

or

ATAG Report Tool User Manual

or some such.

I think do not hold up June publication for this, and keep it fairly high priority for addressing soon after. Also, maybe we will learn what other questions users have that we might want to add to that section. :-)