Document dropck

We currently don't document the rather subtle dropck rule, which ensures that destructors parameterized over lifetimes are safe.

dropck was defined in [RFC 0769](https://github.com/rust-lang/rfcs/blob/master/text/0769-sound-generic-drop.md), and updated in [RFC 1238](https://github.com/nox/rust-rfcs/blob/master/text/1238-nonparametric-dropck.md).