According to the following descriptions for the timeout of suspend, they say that the command does NOT return before the completion of suspend.
Actually, the suspend operation immediately returns to prompt regardless of standalone or domain mode.
It seems that it need to consider to change the description or the behavior.

• standalone mode
  ~~~
  [standalone@localhost:9990 /] :read-operation-description(name=suspend)
  "operation-name" => "suspend",
  "request-properties" => {"timeout" => {
  "description" => "The timeout in seconds that a suspend operation will wait for the suspend operation to complete before returning. If this timeout passes the suspend operation returns, it does not resume normal operations.",
  ~~~

• domain mode
  ~~~
  [domain@localhost:9990 /] :read-operation-description(name=suspend-servers)
  "operation-name" => "suspend-servers",
  request-properties" => {"timeout" => {
  "Timeout in seconds. If this is zero the operation will return immediately, -1 means that it will wait indefinitely. Note that the operation will not roll back if the timeout is exceeded, it just means that not all current requests completed in the specified timeout.",
  ~~~