@bem-react/di add docs about registries for modifers

**not work**
```ts
// Block/_mod/Block_mod@desktop
import { withMod as withModBase } from './mod';
import { blockModRegistry } from './Block_mod.registry/desktop';
import { withRegistry } from '@bem-react/di';

export const withMod = withRegistry(blockModRegistry)(withModBase);
```

**Good**
```ts
// Block/_mod/Block_mod@desktop
import { withMod as withModBase } from './mod';
import { blockModRegistry } from './Block_mod.registry/desktop';
import { withRegistry } from '@bem-react/di';
import { compose } from '@bem-react/core';

export const withMod = compose(withRegistry(blockModRegistry), withModBase);
```