turbo-tasks-malloc: address review feedback on #97761 (#98153)
### What?
Follow-up to #97761, which was auto-submitted before @bgw's review
comments were addressed. Comment-only changes; no behavior change.
- Name the nulled-out out-params in the `mi_process_info` call, applying
the suggestion as written.
- Trim the `memory_usage` doc comment to a brief description plus a link
to the mimalloc docs. `mi_process_info` is absent from mimalloc's
published doxygen docs, so the link points at the `libmimalloc-sys`
rustdoc, which reproduces the header's table describing each figure. It
is a plain URL rather than an intra-doc link because `libmimalloc-sys`
is an optional dependency — an intra-doc link fails to resolve under
`--no-default-features`.
- Trim the test's doc comment to the single suggested line.
The `#[global_allocator]` suggestion matched the existing code exactly,
so nothing to change there. Left the `vec!` in the test alone, per "this
is fine because every test in this crate should expect this global
allocator".
While in the same doc comment, dropped the `[`self::counter`]` link,
which was raising a `private_intra_doc_links` warning.
### How?
Verified with `cargo fmt --check`, `cargo clippy -p turbo-tasks-malloc
--all-targets`, `cargo test -p turbo-tasks-malloc` (6 passed), and
`cargo doc` under `RUSTDOCFLAGS="-D warnings"` with both default and
`--no-default-features`.
