api.md: document preauthorized and tags fields

Updates https://github.com/tailscale/tailscale/issues/2120
Updates https://github.com/tailscale/tailscale/issues/4571
Updates https://github.com/tailscale/tailscale/issues/1369

Signed-off-by: Denton Gentry <dgentry@tailscale.com>

api.md

@@ -819,7 +819,11 @@ Supply the tailnet in the path.
     "devices": {
       "create": {
         "reusable": false,
-        "ephemeral": false
+        "ephemeral": false,
+        "preauthorized": false,
+        "tags": [
+          "tag:example"
+        ]
       }
     }
   }
@@ -843,7 +847,9 @@ echo '{
     "devices": {
       "create": {
         "reusable": false,
-        "ephemeral": false
+        "ephemeral": false,
+        "preauthorized": false,
+        "tags": [ "tag:example" ]
       }
     }
   }
@@ -859,7 +865,7 @@ Response:
 	"key":          "tskey-k123456CNTRL-abcdefghijklmnopqrstuvwxyz",
 	"created":      "2021-12-09T23:22:39Z",
 	"expires":      "2022-03-09T23:22:39Z",
-	"capabilities": {"devices": {"create": {"reusable": false, "ephemeral": false}}}
+	"capabilities": {"devices": {"create": {"reusable": false, "ephemeral": false, "preauthorized": false, "tags": [ "tag:example" ]}}}
 }
 ```