Clarify M-SMALLER-CRATES is mostly orthogonal to whether users should see the smaller crates.

... in other words, it's perfectly normal in rust to have `foo` re-exporting `foo_x` and `foo_y` as an umbrella, and rightfully expecting that users will not deal with `foo_x` and `foo_y` directly (and if they do and run into issues, e.g., around macros) it's just a not supported use case).