Add comment that field projectin also works with mutable fields

src/libcore/pin.rs

@@ -171,11 +171,12 @@
 //!
 //! # Projections and Structural Pinning
 //!
-//! One interesting question arises when considering the interaction of pinning and
-//! the fields of a struct. When can a struct have a "pinning projection", i.e.,
-//! an operation with type `fn(Pin<&Struct>) -> Pin<&Field>`?
-//! In a similar vein, when can a generic wrapper type (such as `Vec<T>`, `Box<T>`, or `RefCell<T>`)
-//! have an operation with type `fn(Pin<&Wrapper<T>>) -> Pin<&T>`?
+//! One interesting question arises when considering the interaction of pinning
+//! and the fields of a struct. When can a struct have a "pinning projection",
+//! i.e., an operation with type `fn(Pin<&Struct>) -> Pin<&Field>`?  In a
+//! similar vein, when can a generic wrapper type (such as `Vec<T>`, `Box<T>`,
+//! or `RefCell<T>`) have an operation with type `fn(Pin<&Wrapper<T>>) ->
+//! Pin<&T>` (or similarly `fn(Pin<&mut Wrapper<T>>) -> Pin<&mut T>`)?
 //!
 //! Having a pinning projection for some field means that pinning is "structural":
 //! when the wrapper is pinned, the field must be considered pinned, too.