Clarify object type documentation

Craig pointed out this is a bit confusing when you don't have all the
fields.  Now we say so, but still include the type's description in case
it's useful.

Fixes #37.

generate/testdata/queries/QueryWithAlias.graphql.go

@@ -16,6 +16,9 @@ type QueryWithAliasResponse struct {
 	User QueryWithAliasUser `json:"User"`
 }
 
+// QueryWithAliasUser includes the requested fields of the GraphQL type User.
+// The GraphQL type's documentation follows.
+//
 // A User is a user!
 type QueryWithAliasUser struct {
 	// id is the user's ID.