Swap getDerived and shouldUpdate in lifecycle diagram (#255)

Also add a note about their interaction to the API reference

docs/api-reference.md

@@ -608,4 +608,7 @@ end
 As with `setState`, you can set use the constant `Roact.None` to remove a field from the state.
 
 !!! note
-	`getDerivedStateFromProps` is a *static* lifecycle method. It does not have access to `self`, and must be a pure function.
+	`getDerivedStateFromProps` is a *static* lifecycle method. It does not have access to `self`, and must be a pure function.
+
+!!! caution
+	`getDerivedStateFromProps` runs before `shouldUpdate` and any non-nil return will cause the state table to no longer be shallow-equal. This means that a `PureComponent` will rerender even if nothing actually changed. Similarly, any component implementing both `getDerivedStateFromProps` and `shouldUpdate` needs to do so in a way that takes this in to account.

docs/images/lifecycle.gv

@@ -25,7 +25,7 @@ digraph G {
 		<br /><br />
 		<font face="monospace">Roact.update</font>
 	>];
-	update -> shouldUpdate -> getDerivedStateFromProps -> willUpdate -> render -> updated -> didUpdate;
+	update -> getDerivedStateFromProps -> shouldUpdate -> willUpdate -> render -> updated -> didUpdate;
 
 	unmounted [style="rounded", color="#000000", label=<<i>Instances Destroyed</i>>];
 	unmount [style="rounded", color="#000000", label=<