Warn on unknown keys in lint table entries

[Alexendoo]

Problem

In rust-lang/rust-clippy#11751 a user had

[lints.clippy]
doc_markdown = { level = "deny", doc_valid_idents = ["GHz", ".."] }

Expecting the doc_valid_idents to set the clippy configuration of the same name, however it is silently ignored by cargo

Steps

No response

Possible Solution(s)

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Notes

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Version

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[epage]

Yeah, we should add a unused_keys warning to these.

[epage]

btw allowing configuration in there is a future possibility though previously a clippy team member said they wouldn't use it.

[Alexendoo]

The tool-wide section from that link looks like it would be a better fit for us yeah