This fixes a limitation that prevented me from adding it to Cargo.
The rust compiler-docs bundle is built by running multiple `cargo`
commands in succession, and I want to support that, so I'm stuck
putting the doc parts all in one directory, so that subsequent
`cargo` runs can pick up the previous runs' data. It's less clean,
but it should still be usable in hermetic build environments if
you give every crate its own directory (which you needed to do
before, oddly enough).

rustdoc: make mergeable crate info more usable

Part of rust-lang#130676

Adds documentation and a feature change aimed at making this usable without breaking backwards compat.

cc `@EtomicBomb`

Rollup of 3 pull requests

Successful merges:

 - #148122 (Make deref_nullptr deny by default instead of warn)
 - #148234 (rustdoc: make mergeable crate info more usable)
 - #149255 (Use `let...else` consistently in user-facing diagnostics)

r? `@ghost`
`@rustbot` modify labels: rollup

Rollup of 6 pull requests

Successful merges:

 - #148234 (rustdoc: make mergeable crate info more usable)
 - #149201 (Add suggest alternatives for Out-of-range \x escapes)
 - #149208 ([rustdoc] Make more functions return `fmt::Result` and reduce number of `.unwrap()` calls)
 - #149252 (miri: use `tikv-jemalloc-sys` from sysroot)
 - #149255 (Use `let...else` consistently in user-facing diagnostics)
 - #149275 (Fix missing double-quote in `std::env::consts::OS` values)

r? `@ghost`
`@rustbot` modify labels: rollup

Rollup merge of #148234 - notriddle:doc-cci, r=GuillaumeGomez

rustdoc: make mergeable crate info more usable

Part of #130676

Adds documentation and a feature change aimed at making this usable without breaking backwards compat.

cc ``@EtomicBomb``