CORE-13335: Fix examples that assume invalid data cannot be ALIVE

[samuelraeburn]

Summary

Update the keyed_data example that assumes that !valid_data means an instance cannot be ALIVE.

Details and comments

The introduction of Instance State Consistency means that a sample without valid_data can be in any instance state.
The following code is no longer correct:

if (!valid_data)
     if (instance_state == NOT_ALIVE_DISPOSED) {
         // handle disposed
     } else {
         // handle NOT_ALIVE_NO_WRITERS
     }

Checks

• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.