Add note about the store retaining the query watcher

[designatednerd]

Trying to add a bit more information about what's happening until I can get around to fixing #516.

[designatednerd]

@AnthonyMDev @koenpunt @wnagrodzki would appreciate feedback on the language here

[AnthonyMDev]

Glad we're adding some docs to this. Thanks Ellen. Here's my suggestion for the wording.

[wnagrodzki]

Thank you. I think that's perfect :)

What do you think about updating this tutorial page with as well? Maybe replacing kill of with mention about cancel() method and memory leak will draw a bit more attention to this issue for newcomers.

Remember to kill off watchers when their parent object is deallocated! This is not (presently) done automatically.

[designatednerd]

@wnagrodzki Yep great point.