rephrase "instances of any screen size"; other fixes (#11063)

_Description of what this PR is changing or adding, and why:_

"instances of any screen size" doesn't quite make sense. What's an
"instance", and how can it have a "screen size"? For the purposes of
add-to-app a _Flutter view_ is embedded into the app, and it indeed [has
size](https://api.flutter.dev/flutter/dart-ui/FlutterView/physicalSize.html).

I also digested what "hybrid navigation stack" and "partial-screen
views" mean. They are not self-explanatory.

src/content/add-to-app/index.md

@@ -8,7 +8,7 @@ description: Adding Flutter as a library to an existing Android or iOS app.
 
 If you are writing a new application from scratch, it is easy to [get started][]
 using Flutter. But what if you already have an app that's not written in
-Flutter, and it's not practical to start from scratch?
+Flutter, and it's impractical to start from scratch?
 
 For those situations, Flutter can be integrated into your existing application
 piecemeal, as a module. This feature is known as "add-to-app". The module can be
@@ -32,10 +32,17 @@ Flutter supports two flavors of add-to-app:
  In this mode there's only one Dart program and all views and widgets can share
  objects.
 
-Add-to-app supports integrating multiple instances of any screen size.
-This can help scenarios such as a hybrid navigation stack with mixed
-native and Flutter screens, or a page with multiple partial-screen Flutter
-views.
+Add-to-app supports integrating multiple Flutter views of any size, supporting
+various use-cases. Two of the most common use-cases are:
+
+* **Hybrid navigation stacks**: an app is made of multiple screens, some of
+ which are rendered by Flutter, and others by another framework. The user can
+ navigate from one screen to another freely, no matter which framework is used
+ to render the screen.
+* **Partial-screen views**: a screen in the app renders multiple widgets, some
+ of which are rendered by Flutter, and others by another framework. The user
+ can scroll and interact with any widget freely, no matter which framework is
+ used to render the widget.
 
 ## Supported features
 
@@ -171,6 +178,7 @@ see our API usage guides at the following links:
 
 Mobile limitations:
 
+* Multi-view mode is not supported (multi-engine only).
 * Packing multiple Flutter libraries into an
  application isn't supported.
 * Plugins that don't support `FlutterPlugin` might have unexpected
@@ -180,7 +188,7 @@ Mobile limitations:
 
 Web limitations:
 
-* Multi-engine mode is not supported.
+* Multi-engine mode is not supported (multi-view only).
 * There's no way to completely "shutdown" the Flutter engine. The app can remove
  all the [FlutterView][] objects and make sure all data is garbage collected
  using normal Dart concepts. However, the engine will remain warmed up, even if