add note to whatsnew

Author: wholmgren

docs/sphinx/source/whatsnew/v0.4.2.txt

@@ -39,6 +39,9 @@ Enhancements
 * Added more attributes to ModelChain, PVSystem, and Location printed
   representations. (:issue:`254`)
 * Added name attribute to ModelChain and PVSystem. (:issue:`254`)
+* Restructured API section of the documentation so that there are
+  separate pages for each function, class, or method. (:issue:`258`)
+
 
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