Add a doctest for the btreemap's entry method.

This is an updated version of #19711. The merge and subsequent rebase on that branch were more trouble than they were worth, so I am just resubmitting the relevant change here.

If this PR is accepted, then #19711 can be closed.

/cc @Gankro 
@Gankro Is it appropriate to do something as simple as:

```rust
assert_eq!(*count.get("a").unwrap(), 3u);
```