json: Clarify that this package is not interesting to import

json/doc.go

@@ -5,4 +5,8 @@
 // This is not a generic JSON parser. Instead, it deals with the mapping from
 // the JSON information model to the HCL information model, using a number
 // of hard-coded structural conventions.
+//
+// In most cases applications will not import this package directly, but will
+// instead access its functionality indirectly through functions in the main
+// "hcl" package and in the "hclparse" package.
 package json