Turf/Buffer - More precise documentation of supported units for buffering

[marcjansen]

This changes the documentation of the buffer module to list all supported units.

lengthToUnits (which is used when buffering) supports more than just miles, kilometers, and degrees. I am not sure if this was not documented on purpose.

I also wasn't sure whether I was supposed to commit the changed README.md as well.

Please review.

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