docs: add background on userData and sessionData

docs/api/app.md

@@ -635,11 +635,18 @@ Returns `string` - The current application directory.
     * `%APPDATA%` on Windows
     * `$XDG_CONFIG_HOME` or `~/.config` on Linux
     * `~/Library/Application Support` on macOS
-  * `userData` The directory for storing your app's configuration files, which by
-    default it is the `appData` directory appended with your app's name.
+  * `userData` The directory for storing your app's configuration files, which
+    by default is the `appData` directory appended with your app's name. By
+    convention files storing user data should be written to this directory, and
+    it is not recommended to write large files here because some environments
+    may backup this directory to cloud storage.
   * `sessionData` The directory for storing data generated by `Session`, such
     as localStorage, cookies, disk cache, downloaded dictionaries, network
-    state, devtools files. By default this points to `userData`.
+    state, devtools files. By default this points to `userData`. Chromium may
+    write very large disk cache here, so if your app does not rely on browser
+    storage like localStorage or cookies to save user data, it is recommended
+    to set this directory to other locations to avoid polluting the `userData`
+    directory.
   * `temp` Temporary directory.
   * `exe` The current executable file.
   * `module` The `libchromiumcontent` library.