Clarify when dependencies are linked

[dhardy]

The reference documents that exern crate has some documentation on linkage of dependencies, but appears to relate to Edition 2015 (or at least not mention linking without extern crate).

We are considering use of extern "Rust" and #[no_mangle] for custom backends in getrandom (standalone demo), however before proceeding would like to know:

1. if we can rely on this poorly documented aspect of the language
2. why "multiple definition" errors are not always present when declaring multiple backend libs with extern crate

[josephlr]

I think this is mainly an issue of documenting @alexcrichton's comment here: #64402 (comment)

To summarize: Cargo dependencies and the --extern rustc flag just make crates candidates for linking, you have to actually use the crate to link it.