The JSDoc for turn.union at https://github.com/Turfjs/turf/blob/master/packages/turf-union/index.ts says:

• Takes two {@link (Multi)Polygon(s)} and returns a combined polygon. If the input polygons are not contiguous, this function returns a {@link MultiPolygon} feature.

However the online documentation at http://turfjs.org/docs/#union says:

Takes two or more polygons

which is wrong, as union only takes two polygons, as two arguments, not any number of polygons as the live documentation suggests.

I'm not sure where I need to place the PR to fix this, given it's already fixed in master.