Add more detail to with_function doc

Author: TomBebbington

src/engine.rs

@@ -97,6 +97,9 @@ dispose!{JitEngine, LLVMOpaqueExecutionEngine, LLVMDisposeExecutionEngine}
 impl<'a> JitEngine {
     /// Run the closure `cb` with the machine code for the function `function`.
     ///
+    /// If the function takes multiple arguments, these should be wrapped in a tuple due to 
+    /// the limitations of Rust's type system.
+    ///
     /// This will check that the types match at runtime when in debug mode, but not release mode.
     /// You should make sure to use debug mode if you want it to error when the types don't match.
     pub fn with_function<C, A, R>(&self, function: &'a Function, cb: C) where A:Compile<'a>, R:Compile<'a>, C:FnOnce(extern "C" fn (A) -> R) {