fde38f1174
Document that CStr::as_ptr returns a type alias Rustdoc resolves type aliases too eagerly #15823 which makes the [std re-export](https://doc.rust-lang.org/stable/std/ffi/struct.CStr.html#method.as_ptr) of `CStr::as_ptr` show `i8` instead of `c_char`. To work around this I've added info about `c_char` in the method's description. BTW, I've also added a comment to what-not-to-do example in case someone copypasted it without reading the surrounding text.