Document configuring relay runtime globally (#4906)

Summary:
In helping an internal team onboard I realized the ConnectionInterface runtime config is not documented. This aims to fill that gap and also document our other global runtime config: FeatureFlags.

Pull Request resolved: #4906

Test Plan: ![Screenshot 2025-02-03 at 4 50 37 PM](https://github.com/user-attachments/assets/43383c4c-aada-474c-9442-77978376a0e9)

Reviewed By: lynnshaoyu

Differential Revision: D69088021

Pulled By: captbaritone

fbshipit-source-id: 3722a429ee7f903e44ba7a11ace08c98964110b3

Author: captbaritone

website/docs/api-reference/relay-runtime/runtime-configuration.md

@@ -0,0 +1,52 @@
+---
+id: runtime-config
+title: Runtime Configuration
+slug: /api-reference/runtime-config/
+description: Configuring the Relay Runtime
+keywords:
+  - feature flags
+  - configuration
+---
+
+## ConnectionInterface
+
+If your server's implementation of the Connection Spec differs from the default interface you will need to configure the Relay Runtime to expect the connection type and field names used in your schema. This is done by updating the global ConnectionInterface instance exported by Relay:
+
+:::note
+You will also need to update your Relay Compiler Config with these same values.
+:::
+
+```ts title="/src/ConfigureRelay.ts"
+import { ConnectionInterface } from 'relay-runtime';
+
+// Note: This must match the values configured in the Relay compiler config.
+ConnectionInterface.inject({
+  END_CURSOR: 'end_cursor',
+  HAS_NEXT_PAGE: 'has_next_page',
+  HAS_PREV_PAGE: 'has_previous_page',
+  START_CURSOR: 'start_cursor',
+  PAGE_INFO: 'page_info',
+  NODE: 'node',
+  CURSOR: 'cursor',
+  EDGES: 'edges',
+  PAGE_INFO_TYPE: 'PageInfo',
+});
+```
+
+## Feature Flags
+
+:::warning
+Feature Flags are used for enabling and configuring unstable Relay features, **regular use of Relay should not need to modify runtime feature flags**. They are documented here for purely informational purposes
+:::
+
+Relay has a number of runtime options called "Feature Flags". In general, these are used for enabling experimental features which are not yet stable and thus not yet enabled by default.
+
+Feature flags in the Relay Runtime are implemented as a global mutable object. To set/configure a feature flag, import that object and mutate it. If you do this in the module scope, the updates will apply before Relay looks at them.
+
+```ts title="/src/ConfigureRelay.ts"
+import { RelayFeatureFlags } from 'relay-runtime';
+
+RelayFeatureFlags.ENABLE_SOME_EXPERIMENT = true;
+```
+
+You can find the full list of feature flags [here](https://github.com/facebook/relay/blob/203d8b10e9144a37466b8a72edbe6add48f64e7d/packages/relay-runtime/util/RelayFeatureFlags.js#L4), but keep in mind that **feature flags may change arbitrarily between versions of Relay**.

website/sidebars.js

@@ -219,6 +219,7 @@ module.exports = {
             'api-reference/relay-runtime/request-subscription',
             'api-reference/relay-runtime/observe-fragment',
             'api-reference/relay-runtime/wait-for-fragment-data',
+            'api-reference/relay-runtime/runtime-config',
           ],
         },
         {