Rollup merge of #39174 - rspeer:iter-nth-doc-fix, r=alexcrichton

Rollup merge of #39174 - rspeer:iter-nth-doc-fix, r=alexcrichton

Fix a misleading statement in `Iterator.nth()`

The `Iterator.nth()` documentation says "Note that all preceding elements will be consumed". I assumed from that that the preceding elements would be the *only* ones that were consumed, but in fact the returned element is consumed as well.

The way I read the documentation, I assumed that `nth(0)` would not discard anything (there are 0 preceding elements, and maybe it just peeks at the start of the iterator somehow), so I added a sentence clarifying that it does. I also rephrased it to avoid the stunted "i.e." phrasing.