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 
You should be able to even do:

```rust
assert_eq!(count["a"], 3);
```