Improve select_nth_unstable documentation clarity

[mgsloan]

• Instead uses before and after variable names in the example
  where greater and lesser are flipped.

• Uses <= and >= instead of "less than or equal to" and "greater
  than or equal to" to make the docs more concise.

• General attempt to remove unnecessary words and be more precise. For
  example it seems slightly wrong to say "its final sorted position",
  since this implies there is only one sorted position for this element.

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