Highlight difference in traits' abstraction level

Short descriptions for NumOps sort of blended in with NumRef and RefNum.
This suggests to clarify the difference a little bit more.

Author: sshilovsky

src/lib.rs

@@ -95,7 +95,7 @@ pub trait Num: PartialEq + Zero + One + NumOps {
     fn from_str_radix(str: &str, radix: u32) -> Result<Self, Self::FromStrRadixErr>;
 }
 
-/// The trait for types implementing basic numeric operations
+/// Generic trait for types implementing basic numeric operations
 ///
 /// This is automatically implemented for types which implement the operators.
 pub trait NumOps<Rhs = Self, Output = Self>:
@@ -123,14 +123,16 @@ impl<T, Rhs, Output> NumOps<Rhs, Output> for T where
 pub trait NumRef: Num + for<'r> NumOps<&'r Self> {}
 impl<T> NumRef for T where T: Num + for<'r> NumOps<&'r T> {}
 
-/// The trait for references which implement numeric operations, taking the
+/// The trait for `Num` references which implement numeric operations, taking the
 /// second operand either by value or by reference.
 ///
-/// This is automatically implemented for types which implement the operators.
+/// This is automatically implemented for all types which implement the operators. It covers
+/// every type implementing the operations though, regardless of it being a reference or
+/// related to `Num`.
 pub trait RefNum<Base>: NumOps<Base, Base> + for<'r> NumOps<&'r Base, Base> {}
 impl<T, Base> RefNum<Base> for T where T: NumOps<Base, Base> + for<'r> NumOps<&'r Base, Base> {}
 
-/// The trait for types implementing numeric assignment operators (like `+=`).
+/// Generic trait for types implementing numeric assignment operators (like `+=`).
 ///
 /// This is automatically implemented for types which implement the operators.
 pub trait NumAssignOps<Rhs = Self>: