Auto merge of #95024 - koehlma:rustdoc-private-items, r=GuillaumeGomez,camelid,jsha

rustdoc: add 🔒 to items with restricted visibility

This change marks items with restricted visibility with 🔒 when building with `--document-private-items`:

<img width="278" alt="Screen Shot 2022-03-20 at 23 50 24" src="https://user-images.githubusercontent.com/509209/159189513-9e4b09bb-6785-41a5-bfe2-df02f83f8641.png">

There also appears a “Restricted Visibility” tooltip when hovering over the emoji.

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The original PR for reference:

This change makes private items slightly transparent (similar to `unstable` items in rustc):

<img width="272" alt="Screen Shot 2022-03-16 at 22 17 43" src="https://user-images.githubusercontent.com/509209/158692627-a1f6f5ec-e043-4aa2-9352-8d2b15c31c08.png">

I found myself using `--document-private-items` a lot recently because I find the documentation of private internals quite helpful when working on a larger project. However, not being able to distinguish private from public items (see #87785) when looking at the documentation makes this somewhat cumbersome.

This PR addresses the third suggestion of issue #87785 by marking private items typographically. It seems to me that the other suggestions are more involved but this is at least a first step.

A private item is also made slightly transparent in the path displayed in the header of a page:

<img width="467" alt="Screen Shot 2022-03-16 at 22 19 51" src="https://user-images.githubusercontent.com/509209/158692885-0bbd3417-3c0b-486f-b8ab-99c05c6fa7ca.png">

I am looking forward to feedback and suggestions.

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