Treat flexPositive/Negative/boxFlex as unitless styles

tips/06-style-props-value-px.md

@@ -18,11 +18,14 @@ See [Inline Styles](/react/tips/inline-styles.html) for more info.
 
 Sometimes you _do_ want to keep the CSS properties unitless. Here's a list of properties that won't get the automatic "px" suffix:
 
+- `boxFlex`
 - `columnCount`
 - `fillOpacity`
 - `flex`
 - `flexGrow`
+- `flexPositive`
 - `flexShrink`
+- `flexNegative`
 - `fontWeight`
 - `lineClamp`
 - `lineHeight`