docs: mention pydot resolution settings (#514)

* docs: mention pydot resolution settings


---------

Co-authored-by: Albert Latham <[email protected]>

Author: comalice

docs/diagram.md

@@ -59,6 +59,18 @@ As this one:
 ![OrderControl](images/order_control_machine_initial.png)
 
 
+If you find the resolution of the image lacking, you can
+
+```py
+>>> dot.set_dpi(300)
+
+>>> dot.write_png("docs/images/order_control_machine_initial_300dpi.png")
+
+```
+
+![OrderControl](images/order_control_machine_initial_300dpi.png)
+
+
 The current {ref}`state` is also highlighted:
 
 ``` py