magento/devdocs#: Add documentation about @magentoConfigFixture annotation

guides/v2.3/graphql/functional-testing.md

@@ -95,7 +95,7 @@ A fixture consists of two files:
 - The fixture file, which defines the test
 - A rollback file, which reverts the system to the state before the test was run
 
-{:.bs-callout .bs-callout-info}
+{: .bs-callout-info }
 Each fixture should have a corresponding rollback file.
 
 Magento provides fixtures in the `dev/tests/integration/testsuite/Magento/<ModuleName>/_files` directory. Use these fixtures whenever possible. When you create your own fixture, also create a proper rollback.
@@ -196,6 +196,50 @@ $registry->unregister('isSecureArea');
 $registry->register('isSecureArea', false);
 ```
 
+### Fixture configs
+
+Use the `@magentoConfigFixture` annotation to set a custom config value. It supports a `store` scope only.
+
+#### Syntax
+
+```php
+/**
+ * @magentoConfigFixture <store_code>_store <config_key> <config_value>
+ */
+```
+
+where
+- `<store_code>` - Store code. See the `store`.`code` database field value.
+- `<config_key>` - Config key. See `core_config_data`.`path` 
+- `<config_value>` - Config value. See `core_config_data`.`value`
+
+{: .bs-callout-info }
+`@magentoConfigFixture` does not require a roll-back.  
+
+#### Example usage 
+
+The following example sets a store-scoped value `1` for the config key `checkout/options/enable_agreements` for the `default` store in the `GetActiveAgreement()` test: 
+
+```php
+    /**
+     * @magentoConfigFixture default_store checkout/options/enable_agreements 1
+     */
+    public function testGetActiveAgreement()
+    {
+        ...
+    }
+```
+
+`@magentoConfigFixture` performs the following action as a background process before test execution:  
+
+```sql
+INSERT INTO `core_config_data` (scope`, `scope_id`, `path`, `value`)
+VALUES
+	('stores', 1, 'checkout/options/enable_agreements', '1');
+```
+
+The fixture automatically removes the `checkout/options/enable_agreements` config key from the database after the test has been completed.
+
 ## Defining expected exceptions
 
 Your functional tests should include events that cause exceptions. Since your tests expect an exception to occur, set up your tests so that they elicit the proper responses. You can define expected exception messages either in: