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 
Either this `u` can be omitted, or the whole type of `count` can-- I think. Your call.