Rollup merge of #45217 - SimonSapin:alloc-doc, r=steveklabnik

Fix out of date unstable book entries for `alloc_*` features.

The `alloc_jemalloc` crate does not provide a type to use with `#[global_allocator]`, and (according to Alex) `extern crate alloc_jemalloc;` alone became a no-op when `#[global_allocator]` was introduced.