api.md: document using '-' value as default tailnet

Signed-off-by: Will Norris <will@tailscale.com>

api.md

@@ -355,6 +355,13 @@ GET /api/v2/tailnet/alice@gmail.com/...
 curl https://api.tailscale.com/api/v2/tailnet/alice@gmail.com/...
 ```
 
+Alternatively, you can specify the value "-" to refer to the default tailnet of
+the authenticated user making the API call.  For example:
+```
+GET /api/v2/tailnet/-/...
+curl https://api.tailscale.com/api/v2/tailnet/-/...
+```
+
 Tailnets are a top-level resource. ACL is an example of a resource that is tied to a top-level tailnet.
 
 For more information on Tailscale networks/tailnets, click [here](https://tailscale.com/kb/1064/invite-team-members).