[TypeScript Guidelines] Subclients can be exposed as readonly properties typed as FooOperations

Bad code snippet:
```ts
readonly sharedAccessSignature: SharedAccessSignatureOperations;
```

What it should do:
The current guideline suggesting that subclients must be exposed only through `get`-prefixed methods with a `Client` suffix is out of date for this generated code. Our codegen intentionally emits readonly properties typed as `FooOperations`, so the guidance should be updated to reflect the current pattern rather than flagging it as incorrect.

Good code snippet:
```ts
readonly sharedAccessSignature: SharedAccessSignatureOperations;
```

Start from the TypeScript guideline that requires subclients to use get-prefixed methods with a Client suffix. Update the guidance to accept readonly properties typed as FooOperations, and verify that the sharedAccessSignature example is no longer flagged as incorrect.

Written by the indexing model from the issue text.