DOC Add a FAQ entry for async filesystem usage (#3222)

Co-authored-by: Roman Yurchak <rth.yurchak@gmail.com>

docs/usage/faq.md

@@ -51,7 +51,8 @@ For security reasons JavaScript in the browser is not allowed to load local data
 You will run into Network Errors, due to the [Same Origin Policy](https://en.wikipedia.org/wiki/Same-origin_policy).
 There is a
 [File System API](https://wicg.github.io/file-system-access/) supported in Chrome
-but not in Firefox or Safari.
+but not in Firefox or Safari. See {ref}`nativefs-api` for experimental local file system
+support.
 
 For development purposes, you can serve your files with a
 [web server](https://developer.mozilla.org/en-US/docs/Learn/Common_questions/set_up_a_local_testing_server).
@@ -409,3 +410,12 @@ importlib.invalidate_caches() # Make sure Python notices the new .py file
 from mymodule import hello
 hello()
 ```
+
+## Why changes made to IndexedDB don't persist?
+
+Unlike other filesystems, IndexedDB (pyodide.FS.filesystem.IDBFS) is an asynchronous filesystem.
+This is because browsers offer only asynchronous interfaces for IndexedDB.
+So in order to persist changes, you have to call
+[`pyodide.FS.syncfs()`](https://emscripten.org/docs/api_reference/Filesystem-API.html#FS.syncfs).
+See [Emscripten File System API](https://emscripten.org/docs/api_reference/Filesystem-API.html#persistent-data)
+for more details.

docs/usage/file-system.md

@@ -58,6 +58,8 @@ pyodide.runPython("import os; print(os.listdir('/mnt'))");
 // ==> The list of files in the Node working directory
 ```
 
+(nativefs-api)=
+
 # (Experimental) Using native file system in the browser
 
 You can access native file system from the browser using the