BufferGeometry documentation suggestion: drawRange for indices vs positions

Discussion
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Before (if/when) Idea: restore `drawMode` in some form or other, to avoid user bugs gets implemented, we should at least add a note to BufferGeometry docs:

For the .index property:

If an index exists, the geometry is drawn with gl.drawElements and the drawRange is the set of indices to draw, otherwise gl.drawArrays is used and the drawRange is the set of positions to draw.

And for the .setDrawRange method:

If an index is present, this set the range of indices to draw, otherwise without an index this sets the range of positions to draw.

where drawRange and index occurrences are links to those properties.