Explicitly state implicit timeout unit

[titusfortner]

In chapter 12. Elements, section 12.5 Interaction:

Wait in an implementation-specific way up to the session implicit wait timeout for element to become interactable.

pageLoad says:

Start a timer. If this algorithm has not completed before timer reaches the session’s session page load timeout in milliseconds, return an error with error code timeout.

script says:

If promise is still pending and session script timeout milliseconds is reached, return error with error code script timeout.

I think the unit of the implicit wait value needs to be explicitly stated. I can PR something if this is the right place to define it.