Editorial: Add response stream to Response Section (#1135)

* Add response stream to Response Section

* avoid future tense

Author: robrichard

spec/Section 6 -- Execution.md

@@ -165,11 +165,11 @@ ExecuteMutation(mutation, schema, variableValues, initialValue):
 ### Subscription
 
 If the operation is a subscription, the result is an _event stream_ called the
-"Response Stream" where each event in the event stream is the result of
-executing the operation for each new event on an underlying "Source Stream".
+_response stream_ where each event in the event stream is the result of
+executing the operation for each new event on an underlying _source stream_.
 
 Executing a subscription operation creates a persistent function on the service
-that maps an underlying Source Stream to a returned Response Stream.
+that maps an underlying _source stream_ to a returned _response stream_.
 
 Subscribe(subscription, schema, variableValues, initialValue):
 
@@ -257,9 +257,9 @@ service details should be chosen by the implementing service.
 
 #### Source Stream
 
-A Source Stream is an _event stream_ representing a sequence of root values,
-each of which will trigger a GraphQL execution. Like field value resolution, the
-logic to create a Source Stream is application-specific.
+:: A _source stream_ is an _event stream_ representing a sequence of root
+values, each of which will trigger a GraphQL execution. Like field value
+resolution, the logic to create a _source stream_ is application-specific.
 
 CreateSourceEventStream(subscription, schema, variableValues, initialValue):
 
@@ -294,7 +294,7 @@ operation type.
 
 #### Response Stream
 
-Each event from the underlying Source Stream triggers execution of the
+Each event from the underlying _source stream_ triggers execution of the
 subscription _selection set_ using that event's value as the {initialValue}.
 
 MapSourceToResponseEvent(sourceStream, subscription, schema, variableValues):
@@ -339,7 +339,7 @@ Note: The {ExecuteSubscriptionEvent()} algorithm is intentionally similar to
 
 #### Unsubscribe
 
-Unsubscribe cancels the Response Stream when a client no longer wishes to
+Unsubscribe cancels the _response stream_ when a client no longer wishes to
 receive payloads for a subscription. This in turn also cancels the Source
 Stream, which is a good opportunity to clean up any other resources used by the
 subscription.

spec/Section 7 -- Response.md

@@ -10,7 +10,12 @@ the case that any _field error_ was raised on a field and was replaced with
 
 ## Response Format
 
-A response to a GraphQL request must be a map.
+A GraphQL request returns either a _response_ or a _response stream_.
+
+### Response
+
+:: A GraphQL request returns a _response_ when the GraphQL operation is a query
+or mutation. A _response_ must be a map.
 
 If the request raised any errors, the response map must contain an entry with
 key `errors`. The value of this entry is described in the "Errors" section. If
@@ -35,6 +40,11 @@ Note: When `errors` is present in the response, it may be helpful for it to
 appear first when serialized to make it more clear when errors are present in a
 response during debugging.
 
+### Response Stream
+
+:: A GraphQL request returns a _response stream_ when the GraphQL operation is a
+subscription. A _response stream_ must be a stream of _response_.
+
 ### Data
 
 The `data` entry in the response will be the result of the execution of the