The current description for the refDistance attribute [1] is

A reference distance for reducing volume as source moves further from the listener. The default value is 1. A RangeError exception MUST be thrown if this is set to a negative value.

From the formulas for each DistanceModelType [2] one can deduce that it is the distance at which there is no reduction in volume (gain=1). To me this is not clear from the description above.
For further clarification I suggest adding at least a sentence like

For distances greater than this the volume will be reduced.

If it is just me or you think the formulas in the specification are enough to describe this, feel free to just close & dismiss this issue.

[1] https://webaudio.github.io/web-audio-api/#dom-pannernode-refdistance
[2] https://webaudio.github.io/web-audio-api/#dom-distancemodeltype-linear