Include waiter section in documentation even if there are none

[kdaily]

Is your feature request related to a problem? Please describe.

All clients have a method called get_waiter. The documentation for this indicates to look in the Waiter section of the docs. If a service doesn't have any waiters defined, there is no Waiter section in the docs.

Describe the solution you'd like

Even if there are no waiters, include a waiter section in the docs stating that there are no waiters defined.

This could also be extended to other properties, like paginators.

This was proposed in #2483.