Clean up E0689 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0689.md

@@ -1,13 +1,16 @@
-This error indicates that the numeric value for the method being passed exists
-but the type of the numeric value or binding could not be identified.
+A method was called on an ambiguous numeric type.
 
-The error happens on numeric literals:
+Erroneous code example:
 
 ```compile_fail,E0689
-2.0.neg();
+2.0.neg(); // error!
 ```
 
-and on numeric bindings without an identified concrete type:
+This error indicates that the numeric value for the method being passed exists
+but the type of the numeric value or binding could not be identified.
+
+The error happens on numeric literals and on numeric bindings without an
+identified concrete type:
 
 ```compile_fail,E0689
 let x = 2.0;
@@ -19,8 +22,8 @@ Because of this, you must give the numeric literal or binding a type:
 ```
 use std::ops::Neg;
 
-let _ = 2.0_f32.neg();
+let _ = 2.0_f32.neg(); // ok!
 let x: f32 = 2.0;
-let _ = x.neg();
-let _ = (2.0 as f32).neg();
+let _ = x.neg(); // ok!
+let _ = (2.0 as f32).neg(); // ok!
 ```