Add a doctest for the entry method of BTreeMap.

This isn't very idiomatic "entry" code, to be honest. Generally when you're working with entry you're genuinely uncertain if the value is contained, and have something useful to do in both cases. If you "know" the value is-or-isn't there, you should be using indexing or `insert`.

See the [top-level collection docs](http://doc.rust-lang.org/nightly/std/collections/#entries) for some examples I wrote.