docs(types): explain FilesystemDirectory types (#2663)

core/src/core-plugin-definitions.ts

@@ -590,22 +590,40 @@ export interface FilesystemPlugin extends Plugin {
 export enum FilesystemDirectory {
   /**
    * The Documents directory
+   * On iOS it's the app's documents directory.
+   * Use this directory to store user-generated content.
+   * On Android it's the Public Documents folder, so it's accessible from other apps.
+   * It's not accesible on Android 10 and newer.
    */
   Documents = 'DOCUMENTS',
   /**
    * The Data directory
+   * On iOS it will use the Documents directory
+   * On Android it's the directory holding application files.
+   * Files will be deleted when the application is uninstalled.
    */
   Data = 'DATA',
   /**
    * The Cache directory
+   * Can be deleted in cases of low memory, so use this directory to write app-specific files
+   * that your app can re-create easily.
    */
   Cache = 'CACHE',
   /**
-   * The external directory (Android only)
+   * The external directory
+   * On iOS it will use the Documents directory
+   * On Android it's the directory on the primary shared/external
+   * storage device where the application can place persistent files it owns.
+   * These files are internal to the applications, and not typically visible
+   * to the user as media.
+   * Files will be deleted when the application is uninstalled.
    */
   External = 'EXTERNAL',
   /**
-   * The external storage directory (Android only)
+   * The external storage directory
+   * On iOS it will use the Documents directory
+   * On Android it's the primary shared/external storage directory.
+   * It's not accesible on Android 10 and newer.
    */
   ExternalStorage = 'EXTERNAL_STORAGE'
 }