Rust Guidelines: RequestContent parameters should not be required to be named `content`

Bad code snippet:
```Rust
pub async fn restore_secret(&self, parameters: RequestContent, options: Option) -> Result> {}
```

What it should do:
The APIView guidelines are outdated here. `RequestContent` parameters should not be enforced to use the name `content` going forward, so `parameters` should be accepted as valid.

Good code snippet:
```Rust
pub async fn restore_secret(&self, parameters: RequestContent, options: Option) -> Result> {}
```

Start with the APIView Rust guidelines rule that validates RequestContent parameter names and trace how the current `content` requirement is applied. Update the rule so `parameters` is accepted for RequestContent, then verify that the provided restore_secret example is treated as valid and that existing naming checks still behave as intended.

Written by the indexing model from the issue text.