TOOLS/lua/observe-all: add a warning

This is just natural, but it's also not that obvious.

TOOLS/lua/observe-all.lua

@@ -1,4 +1,8 @@
 -- Test script for property change notification mechanism.
+-- Note that watching/reading some properties can be very expensive, or
+-- require the player to synchronously wait on network (when playing
+-- remote files), so you should in general only watch properties you
+-- are interested in.
 
 local utils = require("mp.utils")