add allow_unused config to missing_docs_in_private_items

[quangcito]

fixes #14413
add allow_unused config to missing_docs_in_private_items for underscored field

changelog: [missing_docs_in_private_items]: add allow_unused config to missing_docs_in_private_items for underscored field

Explaination (quoted from the issue's author): When writing structures that must adhere to a specific format (such as memory mapped I/O structures) there are inevitably fields that are "reserved" by specifications and thus need no documentation. In cases where private docs are preferred but reserved fields need no explanation, having to #[allow/expect] this lint removes the ability to check for otherwise used fields' documentation.

[rustbot]

r? @Jarcho

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