fix: Rust private registry authentication documentation

[i10416]

Changes

Documentation for Rust private crate authentication

This won't change renovate behavior.

Context

• hostType should be either platform or datasource. For Rust crate, datasource is not cargo, but crate
• registryUrls in packageRules for private crate registry require sparse+ prefix.
  • guess from

    renovate/lib/modules/datasource/crate/index.spec.ts

    Line 368 in dc7b8d9

    const res = await getPkgReleases({

hostType is another way to filter rules and can be either a platform such as github and bitbucket-server, or it can be a datasource such as docker and rubygems.
https://docs.renovatebot.com/configuration-options/

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

[rarkins]

I'm really surprised if this works