Fix Custom Prompts documentation for named arguments + include canonical examples / reference.

What is the type of issue?

Documentation is incorrect

What is the issue?

The Custom Prompts documentation seems to be incomplete / outdated:

• https://github.com/openai/codex/blob/main/docs/prompts.md

I know of at least 2 recent PR's that worked on this feature, one of them added/updated these docs, but the second one didn't seem to touch them at all:

• https://github.com/openai/codex/pull/4470
• https://github.com/openai/codex/pull/4474

It would be nice to have a canonical, complete reference (including examples maybe?) of what a custom prompt should look like.

Where did you find it?

https://github.com/openai/codex/blob/main/docs/prompts.md