h3SetToMultiPolygon bug with duplicate hexagon indices

[Pessimistress]

To reproduce:

• Visit https://codepen.io/Pessimistress/pen/ZEWMmeB
• Uncomment line 7, which adds a duplicate hexagon index

Expected

Actual

[nrabinowitz]

At present, it's the responsibility of the caller to ensure that the indexes passed in are unique. Duplicate indexes, invalid indexes, or indexes at multiple resolutions result in undefined behavior. This is primarily a performance-based choice, as validating and de-duping the input set is potentially quite expensive, and is unnecessary if the input set is known to be valid by the caller.

This is not well-documented, though, so at a minimum we should add this warning to the docs.

[nrabinowitz]

It is documented on the main H3 site under h3SetToLinkedGeo: https://h3geo.org/docs/api/regions

But we should definitely document on h3-js as well.