Mention potential sandboxing bugs in INSTALL.

INSTALL

@@ -401,7 +401,10 @@
 
     --enable-sandbox=METHOD
                 This feature is EXPERIMENTAL in the XZ Utils 5.2.x and
-                disabled by default.
+                disabled by default. If you test this, look especially
+                if message translations and locale-specific decimal and
+                thousand separators (e.g. xz --list foo.xz) work the
+                same way as they do without sandboxing.
 
                 There is limited sandboxing support in the xz tool. If
                 built with sandbox support, it's used automatically when