Add docs to C, C++, JS

[robertbastian]

These three HIR backends do not currently produce docs. See the Dart backend for an example.

[YJDoc2]

Hey, is this issue related to adding the doc comments, or the doc comments not actually producing rust docs? I was looking in https://github.com/rust-diplomat/diplomat/tree/main/tool/src/c , and https://github.com/rust-diplomat/diplomat/tree/main/tool/src/js , the source files seem to have doc comments. Is there any other kind of documentation to be added, or am I looking in wrong places?

[Manishearth]

This issue is about adding some form of documentation (comments, sphinx, something) to the code generated by these backends.

This isn't about rustdoc.