Improve vec `partition` and `partitioned` method doc.

Explicitly note in vec `partition` and `partitioned` that the left and
right parts each map to satisfying and non-satisfying elements.
auto merge of #12622 : pnkfelix/rust/fsk-improve-vec-partition-doc, r=huonw

Explicitly note in vec `partition` and `partitioned` that the left and
right parts each map to satisfying and non-satisfying elements.