Add doc string for the auth token command (#2302)

## Changes
The intent of this PR is to clarify that the `databricks auth token`
command is not supported for M2M auth. Fixes:
#1939

## Tests
Manually.

cmd/auth/token.go

@@ -46,6 +46,10 @@ func newTokenCommand(persistentAuth *auth.PersistentAuth) *cobra.Command {
 	cmd := &cobra.Command{
 		Use:   "token [HOST]",
 		Short: "Get authentication token",
+		Long: `Get authentication token from the local cache in ~/.databricks/token-cache.json.
+Refresh the access token if it is expired. Note: This command only works with
+U2M authentication (using the 'databricks auth login' command). M2M authentication
+using a client ID and secret is not supported.`,
 	}
 
 	var tokenTimeout time.Duration