State that built-in mandatory directives must be omitted (#816)

* State that built-in mandatory directives must be omitted

Following on from #815 (comment), this aims to state that built-in directives must be omitted from the SDL.

* Update Section 3 -- Type System.md

Co-authored-by: Lee Byron <lee@leebyron.com>

Author: maraisr

spec/Section 3 -- Type System.md

@@ -1881,6 +1881,10 @@ GraphQL implementations that support the type system definition language should
 provide the `@specifiedBy` directive if representing custom scalar
 definitions.
 
+When representing a GraphQL schema using the type system definition language,
+built in directives (any defined in this specification) should be omitted for
+brevity. Custom directives in use must be specified.
+
 **Custom Directives**
 
 GraphQL services and client tooling may provide additional directives beyond