Merge pull request #169 from ietf-wg-asdf/sdfref-clar

Clarify that result of sdfRef needs to be valid SDF

sdf.md

@@ -1107,9 +1107,14 @@ Name references occur only in specific elements of the syntax of SDF:
 ## sdfRef {#sdfref}
 
 In a JSON map establishing a definition, the keyword `sdfRef` is used
-to copy all of the qualities and enclosed definitions of the referenced definition, indicated
-by the included name reference, into the newly formed definition.
+to copy the qualities and enclosed definitions of the referenced
+definition, indicated by the included name reference, into the newly
+formed definition.
 (This can be compared to the processing of the `$ref` keyword in {{-jso}}.)
+The referenced definition should be such that, after copying and
+applying the additional qualities in the referencing definition, the
+newly built definition is also valid SDF (e.g., the copied qualities
+and definitions are valid in the context of the new definition).
 
 For example, this reference: