add motivation within the example

Author: yaacovCR

spec/Section 3 -- Type System.md

@@ -1450,12 +1450,20 @@ inline fragments (with the exception of the meta-field {\_\_typename}).
 For example, we might define the following types:
 
 ```graphql example
-union SearchResult = Photo | Person
+interface Link {
+  link: Downloadable
+}
 
-interface Downloadable = {
-  url: String
+type SearchResultLink implements Link {
+  link: DownloadableSearchResult
+}
+
+interface Downloadable {
+  url: string
 }
 
+union SearchResult = Photo | Person
+
 intersection DownloadableSearchResult = SearchResult & Downloadable
 
 type Person implements Downloadable {
@@ -1475,6 +1483,11 @@ type SearchQuery {
 }
 ```
 
+Because intersection `DownloadableSearchResult` includes interface
+`Downloadable` as a constraining member type, the possible types of the
+intersection implement the `Downloadable` interface. The `link` field within
+`SearchResultLink` therefore implements the `link` field of interface `Link`.
+
 Just as with unions, the below could be ambiguous and is invalid.
 
 ```graphql counter-example