rustdoc: transitive Drop indicators should be shown

[ishitatsuyuki]

A type can contain types implements Drop but doesn't implement Drop itself; I'll call this "transitive Drop" below.

Such types doesn't explicitly implement drop, but still have destructors run. We should indicate this in some way; for example, Drop has a meaning in futures (cancellation), but we can't see that currently through the docs; documenting transitive Drop will help the understanding.