Improve `select_nth_unstable` documentation clarity

* 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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