Documentation missing for canGoBack()

Current Behavior

According to @satya164 in react-navigation/react-navigation#7814 we should use canGoBack() to guard against going back too much. If we don't, it'll result in a console.error() being emitted (red screen).

canGoBack is an undocumented feature. It's only mentioned in passing in the 'upgrading from 3.x' article where it's mentioned as a feature on the HeaderBackButton, however the function is (also?) available on the navigator object.

Expected Behavior
Typing "canGoBack" in search field in the documentation should yield this result.

How to reproduce

1. Open https://reactnavigation.org/docs
2. Type "canGoBack" in upper right search field

** Other comments **

It may make sense to reference the new documentation for canGoBack() in relation to goBack(), pop(), jumpTo(), and popToTop().

まず、ドキュメントで canGoBack()、goBack()、pop()、jumpTo()、popToTop() への既存の参照を検索します。canGoBack() について、利用可能であることとガードする目的を説明し、関連するナビゲーションメソッドへのリンクを含む、検索可能な項目を追加します。完了条件は、ドキュメントで canGoBack を検索すると新しい説明が返されることです。