C-LINK should encourage use of intra-rustdoc links over manual linking

[ilyvion]

Since rust-lang/rust#43466 (intra-rustdoc links) was stabilized in Rust 1.48, the information in C-LINK feels outdated, as it still suggests linking to other items manually rather than by making use of intra-rustdoc links:

https://github.com/rust-lang/api-guidelines/blob/0e41903686fa805c2e0216d54913109fa79e1383/src/documentation.md#L147-L172