Use the spec wording for "type-system" directives and "executable" directives (#8169)

Be consistent in wording. See
https://discord.com/channels/625400653321076807/862834364718514196/1457146613314359448
for the discussion.

---------

Co-authored-by: Benjie <[email protected]>

Author: martinbonnin

docs/source/schema/directives.md

@@ -67,13 +67,15 @@ If `@deprecated` appears elsewhere in a GraphQL document, it produces an error.
 
 > If you create a [custom directive](#custom-directives), you need to define it (and its valid locations) in your schema. You don't need to define [default directives](#default-directives) like `@deprecated`.
 
-### Schema directives vs. operation directives
+### Type system vs. executable directives
 
-Usually, a given directive appears _exclusively_ in GraphQL schemas or _exclusively_ in GraphQL operations (rarely both, although the spec allows this).
+Usually, a given directive appears _exclusively_ in GraphQL type system documents (schemas) or _exclusively_ in GraphQL executable documents (operations and fragments). They rarely appear in both, although the spec allows this.
 
-For example, among the [default directives](#default-directives), `@deprecated` is a schema-exclusive directive and `@skip` and `@include` are operation-exclusive directives.
+For example, among the [default directives](#default-directives), `@deprecated` is a type-system directive and `@skip` and `@include` are executable directives.
 
-The [GraphQL spec](https://spec.graphql.org/June2018/#sec-Type-System.Directives) lists all possible directive locations. Schema locations are listed under `TypeSystemDirectiveLocation`, and operation locations are listed under `ExecutableDirectiveLocation`.
+The [GraphQL spec](https://spec.graphql.org/June2018/#sec-Type-System.Directives) lists all possible directive locations. Type-system locations are listed under `TypeSystemDirectiveLocation`, and executable locations are listed under `ExecutableDirectiveLocation`.
+
+(The term "operation directive" is ambiguous, and so discouraged in favor of "executable directive". Similarly "type-system directive" is preferred over "schema directive".)
 
 ## Default directives