Fix/update core::ops module documentation w.r.t. operator traits

* Not all traits are part of the prelude anymore
* We switched from pass-by-reference to pass-by-value for most traits
* Add some explanations around pass-by-value traits in the context of
  generic code and additional implementations for reference types.

src/libcore/ops.rs

@@ -13,12 +13,20 @@
 //! Implementing these traits allows you to get an effect similar to
 //! overloading operators.
 //!
-//! The values for the right hand side of an operator are automatically
-//! borrowed, so `a + b` is sugar for `a.add(&b)`.
-//!
-//! All of these traits are imported by the prelude, so they are available in
+//! Some of these traits are imported by the prelude, so they are available in
 //! every Rust program.
 //!
+//! Many of the operators take their operands by value. In non-generic
+//! contexts involving built-in types, this is usually not a problem.
+//! However, using these operators in generic code, requires some
+//! attention if values have to be reused as opposed to letting the operators
+//! consume them. One option is to occasionally use `clone()`.
+//! Another option is to rely on the types involved providing additional
+//! operator implementations for references. For example, for a user-defined
+//! type `T` which is supposed to support addition, it is probably a good
+//! idea to have both `T` and `&T` implement the traits `Add<T>` and `Add<&T>`
+//! so that generic code can be written without unnecessary cloning.
+//!
 //! # Example
 //!
 //! This example creates a `Point` struct that implements `Add` and `Sub`, and then