updates following PR feedback

Author: romanivantsov

spec/Section 3 -- Type System.md

@@ -220,10 +220,6 @@ Likewise, when representing a GraphQL schema using the type system definition
 language, a schema definition should be omitted if it only uses the default root
 operation type names.
 
-Likewise, when representing a GraphQL schema using the type system definition
-language, a schema definition should be omitted if it only uses the default root
-operation type names.
-
 This example describes a valid complete GraphQL schema, despite not explicitly
 including a {`schema`} definition. The {"Query"} type is presumed to be the
 {`query`} root operation type of the schema.

spec/Section 5 -- Validation.md

@@ -1,6 +1,6 @@
 # Validation
 
-GraphQL service does not just verify if a request is syntactically correct, but also
+A GraphQL service does not just verify if a request is syntactically correct, but also
 ensures that it is unambiguous and mistake-free in the context of a given
 GraphQL schema.
 
@@ -13,8 +13,9 @@ Typically validation is performed in the context of a request immediately before
 execution, however a GraphQL service may execute a request without explicitly
 validating it if that exact same request is known to have been validated before.
 For example: the request may be validated during development, provided it does
-not later change. Any client-side or development-time 
-tool should report validation errors and not allow the
+not later change, or a service may validate a request once and memoize the
+result to avoid validating the same request again in the future. Any client-side
+or development-time tool should report validation errors and not allow the
 formulation or execution of requests known to be invalid at that given point in
 time.
 
@@ -577,7 +578,7 @@ fragment conflictingDifferingResponses on Pet {
   - Let {selectionType} be the unwrapped result type of {selection}.
   - If {selectionType} is a scalar or enum:
     - The subselection set of that selection must be empty.
-  - If {selectionType} is an interface, union, object or list of those types:
+  - If {selectionType} is an interface, union, or object:
     - The subselection set of that selection must NOT BE empty.
 
 **Explanatory Text**

spec/Section 6 -- Execution.md

@@ -1,6 +1,6 @@
 # Execution
 
-GraphQL service generates a response from a request via execution.
+A GraphQL service generates a response from a request via execution.
 
 :: A _request_ for execution consists of a few pieces of information:
 
@@ -144,7 +144,8 @@ executing the operation’s top level selection set on the mutation root object
 type. This selection set should be executed serially.
 
 It is expected that the top level fields in a mutation operation perform
-side-effects on the underlying data system. 
+side-effects on the underlying data system. Serial execution of the provided
+mutations ensures against race conditions during these side-effects.
 
 ExecuteMutation(mutation, schema, variableValues, initialValue):