Fixes https://github.com/flutter/flutter/issues/137924
Fixes https://github.com/flutter/flutter/issues/136885

The framework defaults to keeping the widget tree alive and ready to render regardless of application lifecycle events such as showing or hiding a view controller.

Add-to-app developers may be surprised by this, and tend to file bugs asking why `dispose` isn't called when the view is dismissed (or why memory usage is higher than expected after dismissing Flutter UI).

Adds documentation to explain the limitation and what to do instead.

/cc @zanderso @chinmaygarde @jonahwilliams @jason-simmons since we were discussing this in triage.