a+=b vs a=a+b : clarify the difference is not universal

cben · web-flow · commit c95ba001600c · 2017-09-06T09:12:37.000+03:00
Made the text say it depends on class, and rest of explanation talk of lists, not objects.
diff --git a/README.md b/README.md
@@ -858,11 +858,11 @@ a += [5, 6, 7, 8]
 
 #### 💡 Explanation:
 
-*  a += b doesn't behave the same way as a = a + b
+*  `a += b` doesn't always behave the same way as `a = a + b`.  Classes *may* implement the op= operators differently, and lists do this.
 
-* The expression `a = a + [5,6,7,8]` generates a new object and sets `a`'s reference to that new object, leaving `b` unchanged.
+* The expression `a = a + [5,6,7,8]` generates a new list and sets `a`'s reference to that new list, leaving `b` unchanged.
 
-* The expression `a + =[5,6,7,8]` is actually mapped to an "extend" function that operates on the object such that `a` and `b` still point to the same object that has been modified in-place.
+* The expression `a + =[5,6,7,8]` is actually mapped to an "extend" function that operates on the list such that `a` and `b` still point to the same list that has been modified in-place.
 
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