[DOCS] clarify that OS.get_user_data_dir needs project name

[ci skip]

doc/classes/OS.xml

@@ -385,6 +385,10 @@
 			</return>
 			<description>
 				Returns the absolute directory path where user data is written ([code]user://[/code]).
+				On Linux, this is [code]~/.local/share/godot/app_userdata/[project_name][/code], or [code]~/.local/share/[custom_name][/code] if [code]use_custom_user_dir[/code] is set.
+				On macOS, this is [code]~/Library/Application Support/Godot/app_userdata/[project_name][/code], or [code]~/Library/Application Support/[custom_name][/code] if [code]use_custom_user_dir[/code] is set.
+				On Windows, this is [code]%APPDATA%/Godot/app_userdata/[project_name][/code], or [code]%APPDATA%/[custom_name][/code] if [code]use_custom_user_dir[/code] is set.
+				If the project name is empty, [code]user://[/code] falls back to [code]res://[/code].
 			</description>
 		</method>
 		<method name="get_virtual_keyboard_height">