reference: 'inherits' is more clear than 'implies'

Author: tshepang

src/doc/reference.md

@@ -3517,13 +3517,14 @@ more of the closure traits:
 
 * `FnMut`
   : The closure can be called multiple times as mutable. A closure called as
-    `FnMut` can mutate values from its environment. `FnMut` implies
-    `FnOnce`.
+    `FnMut` can mutate values from its environment. `FnMut` inherits from
+    `FnOnce` (i.e. anything implementing `FnMut` also implements `FnOnce`).
 
 * `Fn`
   : The closure can be called multiple times through a shared reference.
     A closure called as `Fn` can neither move out from nor mutate values
-    from its environment. `Fn` implies `FnMut` and `FnOnce`.
+    from its environment. `Fn` inherits from `FnMut`, which itself
+    inherits from `FnOnce`.
 
 
 ### Trait objects