Guide: Add link to FFI explanation

Author: cakebaker

src/doc/guide.md

@@ -5288,9 +5288,9 @@ There are two circumstances where Rust's safety provisions don't work well.
 The first is when interfacing with C code, and the second is when building
 certain kinds of abstractions.
 
-Rust has support for FFI (which you can read about in the [FFI
-Guide](guide-ffi.html)), but can't guarantee that the C code will be safe.
-Therefore, Rust marks such functions with the `unsafe`
+Rust has support for [FFI](http://en.wikipedia.org/wiki/Foreign_function_interface)
+(which you can read about in the [FFI Guide](guide-ffi.html)), but can't guarantee
+that the C code will be safe. Therefore, Rust marks such functions with the `unsafe`
 keyword, which indicates that the function may not behave properly.
 
 Second, if you'd like to create some sort of shared-memory data structure, Rust