message: docs: update feature flag docs
diff: @@ -8,12 +8,6 @@ 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
  -export type FeatureFlagsHooks = {
  - register(name: string): void;
  -};
  -```
  -
  Here's a code sample:
  ```typescript
  @@ -21,8 +15,7 @@ import { createPlugin } from '@backstage/core';
  export default createPlugin({
  id: 'welcome',
  - register({ router, featureFlags }) {
  - // router.registerRoute('/', Component);
  + register({ featureFlags }) {
  featureFlags.register('enable-example-feature');
  },
  });
  @@ -30,41 +23,22 @@ export default createPlugin({
  ## Using with useApi
  -To use it, you'll first need to register the `FeatureFlags` API via
  -`ApiRegistry` in your `apis.ts` in your App:
  -
  -```tsx
  -import {
  - ApiHolder,
  - ApiRegistry,
  - featureFlagsApiRef,
  - FeatureFlags,
  -} from '@backstage/core';
  -
  -const builder = ApiRegistry.builder();
  -builder.add(featureFlagsApiRef, new FeatureFlags());
  -
  -export default builder.build() as ApiHolder;
  -```
  -
  -Then, later, you can directly use it via `useApi`:
  +To inspect the state of a feature flag inside your plugin, you can use the
  +`FeatureFlagsApi`, accessed via the `FeatureFlagsApiRef`. For example:
  ```tsx
  import React, { FC } from 'react';
  import { Button } from '@material-ui/core';
  -import { FeatureFlagState, featureFlagsApiRef, useApi } from '@backstage/core';
  -
  -const ExampleButton: FC<{}> = () => {
  - const flags = useApi(featureFlagsApiRef).getFlags();
  +import { featureFlagsApiRef, useApi } from '@backstage/core';
  - const handleClick = () => {
  - flags.set('enable-example-feature', FeatureFlagState.On);
  - };
  +const ExamplePage: FC<{}> = () => {
  + const featureFlags = useApi(featureFlagsApiRef);
  return (
  - <Button variant="contained" color="primary" onClick={handleClick}>
  - Enable the 'enable-example-feature' feature flag
  - </Button>
  + <div>
  + <MyPluginWidget>
  + { featureFlags.isActive('enable-example-feature') && <ExperimentalPluginWidget> }
  + </div>
  );
  };
  ```
  