[docs] updated with typescript definitions + code sample

docs/reference/createPlugin-feature-flags.md

@@ -3,13 +3,31 @@
 The `featureFlags` object passed to the `register` function makes it possible for plugins to register Feature Flags in Backstage for users to opt into. You can use this to split out logic in your code for manual A/B testing, etc.
 
 ```typescript
-// This needs to be updated.
+export type FeatureFlagsHooks = {
+  registerFeatureFlag(name: FeatureFlagName): void;
+};
 ```
 
 Then, later, if you want to check if the user has enabled them:
 
-```typescript
-// This needs to be updated.
+```tsx
+import React, { FC } from 'react';
+import { Button } from '@material-ui/core';
+import { featureFlagsApiRef, useApi } from '@backstage/core';
+
+const ExampleButton: FC<{}> = () => {
+  const featureFlagsApi = useApi(featureFlagsApiRef);
+
+  const handleClick = () => {
+    featureFlagsApi.enable('enable-example-feature');
+  };
+
+  return (
+    <Button variant="contained" color="primary" onClick={handleClick}>
+      Enable the 'enable-example-feature' feature flag
+    </Button>
+  );
+};
 ```
 
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