Add doc for unstable config `registries.crates-io.protocol`

[weihanglo]

Problem

With #11215, you can control the protocol crates.io uses by setting registries.crates-io.protocol with either sparse or git or unspecified. The doc are currently missing.

Proposed Solution

Documenting the new behaviour in https://github.com/rust-lang/cargo/blob/master/src/doc/src/reference/unstable.md#sparse-registry. Please take the implementation of #11215 as a reference.

Notes

No response

[0xPoe]

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