tsnet: add test and docs for using an exit node

tsnet nodes have long been able to use an exit node by setting the
ExitNodeID pref through LocalClient, but that was neither documented
nor tested.

Add a package doc section (and regenerate the README) showing how to
select an exit node via EditPrefs, and add TestDialThroughExitNode
verifying that a Dial of a non-tailnet IP is routed over WireGuard to
the exit node rather than dialed from the host network.

Fixes #20986

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: Ia1715a628245c52163083803889fd1f475abfab7