doc(watch_group): Fix comments

To sync with 3de00bd4902d20137dad0b551312eceb9a599d98
fix(mock): Timer.isActive should be false after running callback

For non-periodic timers, isActive should be false after running the timer function.

https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/dart-async.Timer#id_isActive
feat(testability): findBindings and findModels should descend into the ShadowDOM

Note that this won't work for you unless you have a chromedriver build
that works with the shadow DOM.