[codex] docs: rename callback example getter

[abhijeetnardele24-hash]

Summary

• rename the Fetcher example method from getObject to fetchObject
• keep the callback parameter guidance unchanged
• make the example read like callback-based async work instead of a getter returning void

Closes #3501.

Validation

• docs-only change; reviewed the markdown diff locally

[Copilot]

Pull request overview

Updates the “Optional Parameters in Callbacks” examples in the Declaration Files Do’s and Don’ts docs to avoid implying that a getter-like method should return void, while keeping the guidance about callback parameter optionality the same.

Changes:

• Renames the example Fetcher method from getObject to fetchObject in both the WRONG and OK snippets.
• Keeps the callback signature guidance unchanged, but makes the example read more like async “fetch” work.

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