Modify mutable deref rule for &mut T

Author: chorman0773

src/expressions.md

@@ -241,7 +241,7 @@ The following expressions can be mutable place expression contexts:
 * [Temporary values].
 * [Fields][field]: this evaluates the subexpression in a mutable place expression context.
 * [Dereferences][deref] of a `*mut T` pointer.
-* Dereference of a variable, or field of a variable, with type `&mut T`.
+* Dereference of a movable place, with type `&mut T`. This includes variables and their fields, as well as temporaries.
   Note: This is an exception to the requirement of the next rule.
 * Dereferences of a type that implements `DerefMut`:
   this then requires that the value being dereferenced is evaluated in a mutable place expression context.