[RFC 231] Capture mode and closure trait rules

I always have to open RFC 231 to check when i need to explain these rules to others 
So i copied these text (with minimal editorial changes) from RFC 231 to Reference so people can read them more easily.

Author: crlf0710

src/expressions/closure-expr.md

@@ -34,9 +34,26 @@ closure can be forced to capture its environment by copying or moving values by
 prefixing it with the `move` keyword. This is often used to ensure that the
 closure's type is `'static`.
 
+A. The capture modes of the free variables referenced by a closure without the `move`
+keyword are determined as follows:
+
+1. Any variable which is closed over and borrowed mutably is by-reference and mutably borrowed.
+
+2. Any variable of a type that does not implement `Copy` which is moved within the closure is captured by value.
+
+3. Any other variable which is closed over is by-reference and immutably borrowed.
+
+B. Free variables referenced by a `move ||` closure are always captured by value.
+
 The compiler will determine which of the [closure
 traits](types.html#closure-types) the closure's type will implement by how it
-acts on its captured variables. The closure will also implement
+acts on its captured variables. 
+
+A. The trait that the unboxed closure implements is `FnOnce` if any variables were moved *out* of the closure; otherwise `FnMut` if there are any variables that are closed over and mutably borrowed; otherwise `Fn`.
+
+B. The value returned by a `move ||` implements `FnOnce`, `FnMut`, or `Fn`, as determined above; thus, for example, `move |x: int, y| x + y` produces an unboxed closure that implements the `Fn(int, int) -> int` trait (and thus the `FnOnce(int, int) -> int` trait by inheritance). 
+
+The closure will also implement
 [`Send`](special-types-and-traits.html#send) and/or
 [`Sync`](special-types-and-traits.html#sync) if all of its captured types do.
 These traits allow functions to accept closures using generics, even though the