Adds note about procs and multiple variable assignment.

doc/tut1.txt

@@ -202,6 +202,12 @@ statement and all the variables will have the same value:
   echo "x ", x  # outputs "x 42"
   echo "y ", y  # outputs "y 3"
 
+Note that declaring multiple variables with a single assignment which calls a
+procedure can have unexpected results: the compiler will *unroll* the
+assignments and end up calling the procedure several times. If the result of
+the procedure depends on side effects, your variables may end up having
+different values! For safety use only constant values.
+
 
 Constants
 =========