Update FFI signature table to use pipe format

Rustdoc doesn't seem like it's converting the old format to a table as we can see here:
http://static.rust-lang.org/doc/master/complement-cheatsheet.html#ffi-(foreign-function-interface)
This new format should fix that and it's also rendered by Github's markdown preview.
auto merge of #13758 : sodaplayer/rust/patch-1, r=alexcrichton

Rustdoc doesn't seem like it's converting the old format to a table as we can see here:
http://static.rust-lang.org/doc/master/complement-cheatsheet.html#ffi-(foreign-function-interface)
This new format should fix that and it's also rendered by Github's markdown preview.