Apply suggestions from code review

Co-authored-by: Benedikt Franke <benedikt@franke.tech>

spec/Section 3 -- Type System.md

@@ -1660,10 +1660,10 @@ fragment SomeFragment on SomeType {
 }
 ```
 
-A directive may be defined as repeatable at any permitted location with the `repeatable` 
-keyword. Repeatable directives often useful when the same directive should be used with 
-different arguments at the one location, especially in cases where additional information 
-need to be provided in a form of directive via a type or a schema extension:
+A directive may be defined as repeatable at any permitted location with the `repeatable`
+keyword. Repeatable directives are often useful when the same directive should be used with
+different arguments at a single location, especially in cases where additional information
+needs to be provided to a type or schema extension via a directive:
 
 ```graphql example
 directive @delegateField(name: String!) repeatable on OBJECT | INTERFACE

spec/Section 4 -- Introspection.md

@@ -418,4 +418,4 @@ Fields
   locations this directive may be placed.
 * `args` returns a List of `__InputValue` representing the arguments this
   directive accepts.
-* `isRepeatable` must return a Boolean which permits using the directive multiple times at the same location.
+* `isRepeatable` must return a Boolean that indicates if the directive may be used repeatedly at a single location.

spec/Section 5 -- Validation.md

@@ -1440,7 +1440,7 @@ query @skip(if: $foo) {
 **Formal Specification**
 
   * For every {location} in the document for which Directives can apply:
-    * Let {directives} be the set of Directives which apply to {location} and not marked as `repeatable`.
+    * Let {directives} be the set of Directives which apply to {location} and are not `repeatable`.
     * For each {directive} in {directives}:
       * Let {directiveName} be the name of {directive}.
       * Let {namedDirectives} be the set of all Directives named {directiveName}