clarification on defer/stream requirement

# Conflicts:
#	spec/Section 3 -- Type System.md

spec/Section 3 -- Type System.md

@@ -2157,9 +2157,9 @@ fragment someFragment on User {
 
 #### @defer Arguments
 
-- `if: Boolean` - When `true`, fragment may be deferred. When `false`, fragment
-  will not be deferred and data will be included in the initial response. If
-  omitted, defaults to `true`.
+- `if: Boolean` - When `true`, fragment _should_ be deferred. When `false`,
+  fragment will not be deferred and data will be included in the initial
+  response. If omitted, defaults to `true`.
 - `label: String` - A unique label across all `@defer` and `@stream` directives
   in an operation. This label should be used by GraphQL clients to identify the
   data from patch responses and associate it with the correct fragments. If
@@ -2188,12 +2188,24 @@ query myQuery($shouldStream: Boolean) {
 
 #### @stream Arguments
 
-- `if: Boolean` - When `true`, field may be streamed. When `false`, the field
-  will not be streamed and all list items will be included in the initial
+- `if: Boolean` - When `true`, field _should_ be streamed. When `false`, the
+  field will not be streamed and all list items will be included in the initial
   response. If omitted, defaults to `true`.
 - `label: String` - A unique label across all `@defer` and `@stream` directives
   in an operation. This label should be used by GraphQL clients to identify the
   data from patch responses and associate it with the correct fragments. If
   provided, the GraphQL Server must add it to the payload.
 - `initialCount: Int` - The number of list items the server should return as
   part of the initial response. If omitted, defaults to `0`.
+
+Note: The ability to defer and/or stream parts of a response can have a
+potentially significant impact on application performance. Developers generally
+need clear, predictable control over their application's performance. It is
+highly recommended that GraphQL servers honor the `@defer` and `@stream`
+directives on each execution. However, the specification allows advanced
+use-cases where the server can determine that it is more performant to not defer
+and/or stream. Therefore, GraphQL clients _must_ be able to process a response
+that ignores the `@defer` and/or `@stream` directives. This also applies to the
+`initialCount` argument on the `@stream` directive. Clients _must_ be able to
+process a streamed response that contains a different number of initial list
+items than what was specified in the `initialCount` argument.