Document observe_value better

[DominicOram]

When working with observe_value it's not always obvious on the edge cases of it e.g.:

• Do I get a value immediately on calling observe_value?
• When do I get a new value? Do I get one if the there's a callback but the value remains the same?

I think the answers to this are both yes but we should make sure and document it in the docstring

Acceptance Criteria

• The docstring of observe_value answers the above questions