Add section about query directives to schema directive docs.

docs/source/schema-directives.md

@@ -661,6 +661,14 @@ class AuthDirective extends SchemaDirectiveVisitor {
 
 Since the `getDirectiveDeclaration` method receives not only the name of the directive but also the `GraphQLSchema` object, it can modify and/or reuse previous declarations found in the schema, as an alternative to returning a totally new `GraphQLDirective` object. Either way, if the visitor returns a non-null `GraphQLDirective` from `getDirectiveDeclaration`, that declaration will be used to check arguments and permissible locations.
 
+## What about query directives?
+
+As its name suggests, the `SchemaDirectiveVisitor` abstraction is specifically designed to enable transforming GraphQL schemas based on directives that appear in your SDL text.
+
+While directive syntax can also appear in GraphQL queries sent from the client, implementing query directives would require runtime transformation of query documents. We have deliberately restricted this implementation to transformations that take place when you call the `makeExecutableSchema` function—that is, at schema construction time.
+
+We believe confining this logic to your schema is more sustainable than burdening your clients with it, though you can probably imagine a similar sort of abstraction for implementing query directives. If that possibility becomes a desire that becomes a need for you, let us know, and we may consider supporting query directives in a future version of these tools.
+
 ## What about `directiveResolvers`?
 
 Before `SchemaDirectiveVisitor` was implemented, the `makeExecutableSchema` function took a `directiveResolvers` option that could be used for implementing certain kinds of `@directive`s on fields that have resolver functions.