Introduced a chapter that describes how to perform callbacks from C to Rust
Further details on channel idea
Update guide-ffi.md

Simplified the first examples to demonstrate callbacks without other threads involved and shortened the elaboration about async callbacks.
auto merge of #11486 : Matthias247/rust/doc, r=cmr

I wrote a chapter for the FFI tutorial that describes how to perform callbacks from C code to Rust and gives some hints about what to consider and synchronization.

I just needed that for my own wrapper and thought it would be helpful for others.