Fix rustdoc HTML rendering

By returning the items to process and storing them in a queue, we were losing
the context that was setup for that item during the recursion. This is an easy
fix, rather than hoisting out the state that it needs.
auto merge of #11030 : cmr/rust/rustdoc_on_fire, r=metajack

By returning the items to process and storing them in a queue, we were losing
the context that was setup for that item during the recursion. This is an easy
fix, rather than hoisting out the state that it needs.