chore(dialog): add comment for MatDialogContainer injection token (#8029)

Author: josephperrott

src/lib/dialog/dialog.ts

@@ -281,6 +281,10 @@ export class MatDialog {
     const injectionTokens = new WeakMap();
 
     injectionTokens.set(MatDialogRef, dialogRef);
+    // The MatDialogContainer is injected in the portal as the MatDialogContainer and the dialog's
+    // content are created out of the same ViewContainerRef and as such, are siblings for injector
+    // purposes.  To allow the hierarchy that is expected, the MatDialogContainer is explicitly
+    // added to the injection tokens.
     injectionTokens.set(MatDialogContainer, dialogContainer);
     injectionTokens.set(MAT_DIALOG_DATA, config.data);
     injectionTokens.set(Directionality, {