Add doc example for `CString::as_bytes_with_nul`.
Add doc example for `CString::as_bytes_with_nul`.

GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,aidancully/rust,aidancully/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,GBGamer/rust