SCUMMVM: add Android storage info

docs/library/scummvm.md

@@ -196,6 +196,30 @@ Options are grouped into **Video**, **Cursor**, **Timing**, and **RetroPad** cat
 | Select   | Virtual Keyboard | Toggle VKBD          |
 | Start    | ScummVM GUI      | Open Launcher        |
 
+## Android storage access
+
+On modern Android versions apps can only access folders explicitly authorized by the user through the system file picker (Storage Access Framework, SAF).
+
+The core can browse both the standard local filesystem and the folders authorized through the frontend (e.g. RetroArch). The starting location of the ScummVM file browser is controlled by the **Browsing mode** core option.
+
+### Browsing mode
+
+Available under the frontend core options (e.g. RetroArch `Quick Menu > Core Options > System`):
+
+  - **Authorized storage**: the file browser starts from a virtual root listing only the folders authorized through the frontend. Default on Android.
+  - **Local filesystem**: the file browser starts from the standard local path.
+
+### Authorizing folders
+
+  - Open the frontend file browser (e.g. in RetroArch, "Load Content") and use the option to open/add a new folder; the system file picker will appear.
+  - Grant access to the folder(s) containing your games and exit the file browser (no need to actually select any content at this time). The authorization is persistent across reboots.
+  - Start the core; the authorized folders will be listed by the ScummVM file browser when **Browsing mode** is set to `Authorized storage`.
+
+### Notes
+
+  - The authorized folder list is read when the core starts. If you authorize new folders while the core is running, restart (reload) the core to make them available.
+  - If **Browsing mode** is `Authorized storage` but no folders have been authorized, the core falls back to the local filesystem and shows a notification.
+
 ## External Links
 
 - [Official ScummVM Website](http://scummvm.org/)