Show unstable status for deprecated items

[bluss]

A deprecated item should still show its unstable status in the docs, otherwise both stable + deprecated and unstable + deprecated items are presented the same way (only the source code tells you the difference).

This is a problem because without the unstable label, doc items will be assumed to be stable by users.

Issue #24617 seems to be similar, but not exactly the same.

[euclio]

I'd like to work on this. Is there some documentation somewhere on how to build/hack on rustdoc?

[mitaa]

I don't think so..

// build just the rustdoc binary, useful for local experiments
make x86_64-unknown-linux-gnu/stage1/bin/rustdoc
// run the src/test/rustdoc tests
make check-stage1-rustdocck

For writing rustdoc tests you need to look at src/etc/htmldocck.py, which is run on the generated documentation, and compiletest and its flags in general.
(using existing tests as guidance is always useful - i.e. rustdoc tests with auxiliary crates need to be marked // ignore-cross-compile)