point.sensitivity should be documented and default to point.r

A point's sensitivity should probably be equal to its radius, such that hovering on the point will fire the interaction no matter how big the `r` value is.

If nothing else, the `point.sensitivity` value should be documented.

[Case example](http://stackoverflow.com/questions/35204689/c3-js-bubble-chart-selection)

Start by reproducing the linked bubble-chart case and locating the point.sensitivity and point.r configuration or documentation in the C3 codebase. Confirm how hovering behaves at different radius values; done means sensitivity defaults to the radius, or the existing setting is clearly documented if that behavior is not adopted.

Written by the indexing model from the issue text.