api.md: update GET tailnet key detail docs to show preauthorized, tags

Fixes #4571

Change-Id: If81471d0d8cb2f659736991ad8612aed2efc174e
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

api.md

@@ -890,9 +890,21 @@ Response:
 ```
 {
   "id": "k123456CNTRL",
-	"created":      "2021-12-09T22:13:53Z",
-	"expires":      "2022-03-09T22:13:53Z",
-	"capabilities": {"devices": {"create": {"reusable": false, "ephemeral": false}}}
+  "created": "2022-05-05T18:55:44Z",
+  "expires": "2022-08-03T18:55:44Z",
+  "capabilities": {
+    "devices": {
+      "create": {
+        "reusable": false,
+        "ephemeral": true,
+        "preauthorized": false,
+        "tags": [
+          "tag:bar",
+          "tag:foo"
+        ]
+      }
+    }
+  }
 }
 ```