Merge pull request #12739 from mhilbrunner/patch-1

[DOCS] OS.get_unique_string(). See #3628.

doc/classes/OS.xml

@@ -376,7 +376,7 @@
 			<return type="String">
 			</return>
 			<description>
-				Returns a unique string.
+				Returns a string that is unique to the device. Currently only works on Android and iOS. Returns empty string on other platforms.
 			</description>
 		</method>
 		<method name="get_unix_time" qualifiers="const">