docs(overlay): expand overview for overlay (#6976)

src/cdk/overlay/overlay.md

@@ -1,3 +1,88 @@
 ### Overlay
 
-A service used to manage overlays.
+The `Overlay` service provides a way to open floating panels on the screen.
+
+### Creating overlays
+Calling `overlay.create()` will return an `OverlayRef` instance. This instance is a handle for
+managing that specific overlay.
+
+The `OverlayRef` _is_ a `PortalHost`- once created, content can be added by attaching a `Portal`.
+See the documentation on portals for further information.
+```ts
+const overlayRef = overlay.create();
+const userProfilePortal = new ComponentPortal(UserProfile);
+overlayRef.attach(userProfilePortal);
+```
+
+### Configuring an overlay
+When creating an overlay, an optional configuration object can be provided.
+```ts
+const overlayRef = overlay.create({
+  height: '400px',
+  width: '600px',
+});
+```
+
+The full set of configuration options can be found in the API documentation.
+
+#### Position strategies
+The `positionStrategy` configuration option determines how the overlay will be positioned on-screen.
+There are two position strategies available as part of the library: `GlobalPositionStrategy` and
+`ConnectedPositionStrategy`.
+
+`GlobalPositionStrategy` is used for overlays that require a specific position in the viewport,
+unrelated to other elements. This is commonly used for modal dialogs and application-level
+notifications.
+
+`ConnectedPositionStrategy` is used for overlays that are positioned relative to some other "origin"
+element on the page. This is commonly used for menus, pickers, and tooltips. When using the
+connected strategy, a set of preferred positions is provided; the "best" position will be selected
+based on how well the overlay would fit within the viewport.
+
+A custom position strategy can be created by implementing the `PositionStrategy` interface.
+Each `PositionStrategy` defines an `apply` method that is called whenever the overlay's position
+should be updated. A custom position strategy can additionally expose any other APIs necessary as
+related to the positioning of the overlay element.
+
+
+#### Scroll strategies
+The `scrollStrategy` configuration option determines how the overlay will react to scrolling outside
+the overlay element. There are four scroll strategies available as part of the library.
+
+`NoopScrollStrategy` is the default option. This strategy does nothing.
+
+`CloseScrollStrategy` will automatically close the overlay when scrolling occurs.
+
+`BlockScrollStrategy` will block page scrolling while the overlay is open. Note that some
+applications may implement special or customized page scrolling; if the `BlockScrollStrategy`
+conflicts with this kind of situation, it can be overriden by re-providing `BlockScrollStrategy`
+with a custom implementation.
+
+`RepositionScrollStrategy` will re-position the overlay element on scroll. Note that this will have
+some performance impact on scrolling- users should weigh this cost in the context of each specific
+application.
+
+
+A custom scroll strategy can be created by implementing the `ScrollStrategy` interface. Each
+strategy will typically inject `ScrollDispatcher` (from `@angular/cdk/scrolling`) to be notified
+of when scrolling takes place. See the documentation for `ScrollDispatcher` for more information
+on how scroll events are detected and dispatched.
+
+### The overlay container
+The `OverlayContainer` provides a handle to the container element in which all individual overlay
+elements are rendered. By default, the overlay container is appended directly to the document body
+so that an overlay is never clipped by an `overflow: hidden` parent.
+
+#### Full-screen overlays
+The `FullscreenOverlayContainer` is an alternative to `OverlayContainer` that supports correct
+displaying of overlay elements in
+[fullscreen mode](https://developer.mozilla.org/en-US/docs/Web/API/Element/requestFullScreen).
+
+`FullscreenOverlayContainer` can be enabled by providing it in your `NgModule`:
+```ts
+@NgModule({
+  providers: [{provide: OverlayContainer, useClass: FullscreenOverlayContainer}],
+  // ...
+})
+export class MyModule { }
+```