chore: Deprecate UNSTABLE_portalContainer in favor for PortalProvider

packages/@react-aria/overlays/docs/PortalProvider.mdx

@@ -34,6 +34,13 @@ Modals, Popovers, Toasts, and Tooltips will portal their overlay element to. Thi
 your app is already portalling other elements to a location other than the `document.body` and thus requires
 your React Aria components to send their overlays to the same container.
 
+Please note that `UNSTABLE_PortalProvider` is considered `UNSTABLE` because it is an escape hatch, and there are
+many places that an application could portal to. Not all of them will work, either with styling, accessibility,
+or for a variety of other reasons. Typically, it is best to portal to the root of the entire application, typically the `body` element,
+outside of any possible overflow or stacking contexts. We envision `UNSTABLE_PortalProvider` being used to group all of the portalled
+elements into a single container at the root of the app or to control the order of children of the `body` element, but you may have use cases
+that need to do otherwise.
+
 ## Props
 
 <PropTable links={docs.links} component={docs.exports.UNSTABLE_PortalProvider} />
@@ -89,7 +96,7 @@ function App() {
             title: 'Toast complete!',
             description: 'Great success.'
           })}>
-          Toast
+          Open Toast
         </Button>
       </UNSTABLE_PortalProvider>
       <div ref={container} style={{height: '70px', width: '200px',  overflow: 'auto'}}>

packages/@react-aria/overlays/src/PortalProvider.tsx

@@ -15,7 +15,7 @@ import React, {createContext, ReactNode, useContext} from 'react';
 export interface PortalProviderProps {
   /** Should return the element where we should portal to. Can clear the context by passing null. */
   getContainer?: () => HTMLElement | null,
-  /** The content of the PortalProvider. Should contain all children that want to portal their overlays to the element returned by the provided getContainer(). */
+  /** The content of the PortalProvider. Should contain all children that want to portal their overlays to the element returned by the provided `getContainer()`. */
   children: ReactNode
 }