add some documentation about *=

docs/builder/ref.md

@@ -257,11 +257,14 @@ my_component.control += my_component.get_group("my_group")
 
 ### `seq`
 
-Control statements are [sequenced][seq] in the order that they appear in a component's control program, represented by a Python list. Let's say we want to sequence the control statements `A`, `B`, and `C`.
+Control statements are [sequenced][seq] in the order that they appear in a component's
+control program, represented by a Python list.
+Let's say we want to sequence the control statements `A`, `B`, and `C`.
 
 ```python
 my_component.control += [A, B, C]
 ```
+It's worth nothing that sequential usage of `+=` will not nest `seq` blocks unless necessary.
 
 ### `par`
 
@@ -271,6 +274,30 @@ For [parallel compositions][par] of control programs, use the `par()` function.
 my_component.control += [par(A, B), C]
 ```
 
+Alternatively, we can construct `par` blocks by using `*=`. To compose the same control program as above, with `A` and `B` in parallel and sequencing this composition before `C` we could write:
+
+```python
+my_component.control *= par(A,B)
+my_component.control += C
+```
+
+### Chaining `seq` and `par`
+
+It is possible to chain together sequential usage of `+=` and `*=` to construct a control program ``sequentially''.
+In this way, the control program's construction in the builder will look similar to the control program in the output Calyx. For example,
+the previous python example in the `par` section will produce a par block of `A` and `B`, which is sequenced before `C`:
+
+```
+seq{
+  par{
+    A;
+    B;
+  }
+  C;
+}
+
+```
+
 ### `if`
 
 See the language reference for [`if`][if].