Fix po4a.conf
fix "warning: deprecated syntax, use `for` keyword now"
Reference: Fix list in Expressions section
auto merge of #18901 : steveklabnik/rust/quickfix, r=alexcrichton

Small copy/paste error from the crates guide.
auto merge of #18922 : japaric/rust/for, r=jakub-

r? @alexcrichton
auto merge of #18924 : cakebaker/rust/fix_list, r=steveklabnik