Add more explanatory text to the C-LINK section

[nicholasbishop]

The new text explains how to add regular markdown links and how to add
link targets. In particular it makes it clear that rustdoc link
targets go at the end of the docstring rather than inline.

[nicholasbishop]

This came out of my confusion in rust-lang/rust#71680. It took me a while to figure out how to fix the links in that docstring because it's quite long, so I didn't notice that the link targets go at the very end.

I'm still a little confused because the link targets in the rust source code don't look like the examples in this doc. See https://doc.rust-lang.org/src/core/cmp.rs.html#194 for example. It shows links to trait methods as [`eq`]: PartialEq::eq, which doesn't match any of the examples given in the C-LINK section.

[KodrAus]

Thanks @nicholasbishop! This looks good to me