docs: Cargo.toml - Revise description of core dep

[polarathene]

Originally a PR to sunfishcode/rustix-dlmalloc, I later realized that project rebases their fork onto alexcrichton/dlmalloc-rs, so I've raised the PR here instead.

Changes:

• cfg-if is related to conditional compilation by platform target. It was originally appended under the two deps associated to the rustc-std-workspace-core, which was a bit misleading on context? Placed above the commentary to avoid that context ambiguity.
• Revised the core comment.
  • The suggested path is no longer present (changed in 2020 with Rust 1.47)
  • Nor was the reference as useful as the associated commit message from Nov 2018 which references upstream for details. I may still be mistaken with my attempt to clarify when it's relevant, but AFAIU it's only relevant when building the std crate? (otherwise I'm not understanding the removal of the prior compiler_builtins associated dep without setting rust-version = "1.89")