Improve function header docs.

I started to lose track of what these functions were supposed to do, which is
important when you're using them in later parts of the assignment.
Part 3: in-progress

This is the basic idea of what needs to be done, but the output isn't yet
matching.
Part 3: Return correct type on empty results

We're supposed to return a numeric vector; I was just returning whatever the
hell `c()` does by default.
Part 3: only correlate complete observations

I thought this was what was already happening, since we filtered out things
that didn't meet the threshold.  But I was getting a bunch of NAs where I was
supposed to get numbers, and so I thought that looked like a reasonable value
to try for `use`.  And whatdya know, it matched the output.