Rollup merge of #27612 - vincentbernat:fix/doc/iter-nth, r=steveklabnik
Rollup merge of #27612 - vincentbernat:fix/doc/iter-nth, r=steveklabnik

The "nth" element can be confusing. In an array context, we know indexes
start from 0 but one may believe this is not the case with "nth". For
example, would `.nth(1)` return the first (1th/1st) or the second
element?  Rephrase a bit to be less confusing.

r? @steveklabnik

pshc/rust,GBGamer/rust,GBGamer/rust,pshc/rust,aidancully/rust,pshc/rust,GBGamer/rust,dwillmer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,graydon/rust,aidancully/rust,pshc/rust,pshc/rust,graydon/rust,aidancully/rust,dwillmer/rust,dwillmer/rust,dwillmer/rust,dwillmer/rust,pshc/rust,dwillmer/rust,pshc/rust,dwillmer/rust,pshc/rust,aidancully/rust,dwillmer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust