[1.50] `./x.py install doc` changed its default location

[cuviper]

I tried this command:

DESTDIR=foo ./x.py install doc

I expected to see files installed to foo/usr/local/share/doc/rust/html

Instead, I see files installed to foo/usr/local/share/doc/html

This worked on Rust 1.49, and I noticed the change in testing the Rust 1.50 pre-release.

I'm going ahead to call this stable-to-stable since 1.50 is imminent.
@rustbot modify labels: +regression-from-stable-to-stable -regression-untriaged

---

The specific change appears to be commit 4050e27 from #81151, a backport of 2caf9bc from #80797, where the default install docdir changed from "share/doc/rust" to just "share/doc". cc @pietroalbini

Also note that config.toml still documents that the default is "share/doc/rust".

[apiraino]

labeling to T-infra as per sibling PR #80797