rustdoc: Tracking issue for `#[doc(masked)]`

[QuietMisdreavus]

FIXME: Write a proper synopsis.

At time of implementation, i'm not sure the utility of this outside the standard library, but in case anyone else wants to prune their documentation, here's the place to be.

Note

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.

A tracking issue is however not meant for large scale discussion, questions, or bug reports.
Instead, open a dedicated issue for the specific matter or post in the #t-rustdoc Zulip channel.

Discussion comments will get marked as off-topic or deleted.
Repeated discussions on the tracking issue may lead to the tracking issue getting locked.

Steps

• Implementation PR
• Stabilization PR

Unresolved questions

Unclear, needs someone to investigate & summarize.

Implementation history

• hide internal types/traits from std docs via new #[doc(masked)] attribute #44026
• add the issue number to doc_masked's feature gate #45024