Clarify "snapped" waypoint

Waypoints can be freeform or snapped to an intersection.

When manually creating a neighborhood boundary, they are snapped by default.

When you've picked an autogenerated boundary, the created waypoints are unsnapped. (Which makes sense - theres no guarantee that the severance vertices of the autogenerated boundary fall on intersections).

There's already some copy explaining this, but apparently it's pretty easy to miss. I also didn't really understand what was meant by "snapped" until I became more familiar with the feature.

This doesn't strike me as super urgent, especially if we address #201