P34: implementing field type is either exact match or of covariant type (graphql#974)

* Fix: implementing field type is either exact match or covariant type

* Match language in IsValidImplimentation

Co-authored-by: Roman Ivantsov <roman.ivantsov@microsoft.com>
Co-authored-by: Lee Byron <lee@leebyron.com>

Author: 3 people

spec/Section 4 -- Introspection.md

@@ -316,9 +316,10 @@ Fields\:
 **Interface**
 
 Interfaces are an abstract type where there are common fields declared. Any type
-that implements an interface must define all the fields with names and types
-exactly matching. The implementations of this interface are explicitly listed
-out in `possibleTypes`.
+that implements an interface must define all the named fields where each
+implementing field type is equal to or a sub-type of (covariant) the interface
+type. The implementations of this interface are explicitly listed out in
+`possibleTypes`.
 
 Fields\: