update vault login docs to cover stdin default (#14336)

website/content/docs/commands/login.mdx

@@ -35,7 +35,28 @@ the returned token is automatically unwrapped unless:
 
 ## Examples
 
-By default, login uses a "token" method:
+By default, login uses a "token" method and reads from stdin:
+
+```shell-session
+$ vault login
+Token (will be hidden):
+Success! You are now authenticated. The token information displayed below
+is already stored in the token helper. You do NOT need to run "vault login"
+again. Future Vault requests will automatically use this token.
+
+Key                  Value
+---                  -----
+token                s.nDj4BB2tK8NaFffwBZBxyIa1
+token_accessor       ZuaObqdTeCHZ4oa9HWmdQJuZ
+token_duration       ∞
+token_renewable      false
+token_policies       ["root"]
+identity_policies    []
+policies             ["root"]
+```
+
+Alternatively, the token may be provided as a command line argument (note that
+this may be captured by shell history or process listings):
 
 ```shell-session
 $ vault login s.3jnbMAKl1i4YS3QoKdbHzGXq