Functions using c_char show misleading "i8" instead of docs

[glandium]

Take for example https://doc.rust-lang.org/std/ffi/struct.CStr.html#method.from_ptr, which says the signature for CStr::from_ptr is:

pub unsafe fn from_ptr<'a>(ptr: *const i8) -> &'a CStr

The real signature uses c_char instead of i8, which is notably different because c_char can be either i8 or u8 depending on the platform. I had to follow the link to the source to make sure it was taking *const c_char and not an actual *const i8, which would have required a conversion.

[jyn514]

i think this is a duplicate of #15823

[GuillaumeGomez]

It'll be fixed once #112792 is stable.