docs: improve example for "remove all stopped containers"
docs: improve example for "remove all stopped containers"

recommend using `docker container prune`, but show an example on
how to combine commands with a bit more context and warnings
about portability/compatibility.

Thanks to Charlie Arehart to do the initial work on this.

Signed-off-by: Sebastiaan van Stijn <64b2b6d12bfe4baae7dad3d018f8cbf6b0e7a044@gone.nl>

thaJeztah/cli,docker/cli,cpuguy83/cli,docker/cli,thaJeztah/cli,cpuguy83/cli