While reviewing kind issues, I noticed there seems to be a regularly
recurring problem with users expecting `kind delete cluster foo` to
fail if there is not a cluster named `foo` present.

We intentionally do not fail in this case. The delete command is
idempotent, and should be able to be run repeatedly without erroring.
This is very useful in clean up scripts, and many users rely on this
behavior.

It also makes things somewhat declarative in that you are saying "I
don't want a cluster named foo", and `kind` helpfully confirms that
there is no cluster named "foo".

To try to avoid some of these repeated issues being filed, this adds a
note to the quick start docs in the "Deleting a Cluster" section to
point out that this is the expected behavior. This also helps new users
to know that they can depend on this behavior if they are scripting
things with kind.

Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>