Clarify that string refs are considered legacy. (#6692)

docs/08.1-more-about-refs.md

@@ -68,7 +68,11 @@ Note that when the referenced component is unmounted and whenever the ref change
 
 ## The ref String Attribute
 
-React also supports using a string (instead of a callback) as a ref prop on any component, although this approach is mostly legacy at this point.
+> Note:
+>
+> Although string refs are not deprecated, they are considered legacy, and will likely be deprecated at some point in the future. Callback refs are preferred.
+
+React also supports using a string (instead of a callback) as a ref prop on any component.
 
 1. Assign a `ref` attribute to anything returned from `render` such as: