Add an error and adjust the terminology slightly about augmented redi…

…recting constructors (#4157)

Adjust the terminology slightly about augmented redirecting constructors

working/augmentation-libraries/feature-specification.md

@@ -855,8 +855,9 @@ It is a compile-time error if:
     replace a declared super constructor._ **(TODO: Why not? We allow
     "replacing implementation", and this is *something* like that.)**
 
-*   The resulting constructor is not valid (has a redirecting initializer and
-    other initializers, multiple `super` initializers, etc).
+*   The resulting constructor is not valid *(it has a redirection as well as
+    some initializer list elements, or it has multiple `super` initializers,
+    etc)*.
 
 *   A non-redirecting constructor augments a constructor which is not
     potentially non-redirecting.
@@ -923,20 +924,21 @@ present), and it may invoke the augmented body by calling
 
 #### Redirecting generative constructors
 
-A redirecting generative constructor marked `augment` adds its redirecting
-initializer to the augmented constructors initializer list.
+A redirecting generative constructor marked `augment` adds its redirection
+to the augmented constructor.
 
 This converts it into a redirecting generative constructor, removing the
 potentially non-redirecting property of the constructor.
 
 It is a compile-time error if:
 
-*   The augmented constructor has any initializers or a body.
+*   The augmented constructor has an initializer list or a body, or it has a
+    redirection.
 
 #### Redirecting factory constructors
 
 A redirecting factory constructor marked `augment` adds its factory redirection
-to the augmented constructor.
+*(e.g., `= C<int>.name`)* to the augmented constructor.
 
 The result of applying the augmenting constructor is a redirecting factory
 constructor with the same target constructor designation as the augmenting
@@ -945,7 +947,7 @@ constructor.
 
 It is a compile-time error if:
 
-*   The augmented constructor has a body.
+*   The augmented factory constructor has a body, or it is redirecting.
 
 #### Extension types
 
@@ -1201,8 +1203,6 @@ declaration ::= 'external'? factoryConstructorSignature
   | constructorSignature (redirection | initializers)?
 ```
 
-**TODO: Define the grammar for the various `augmented` expressions.**
-
 It is a compile-time error if:
 
 *   A declaration marked `augment` is also marked `external`. **(TODO: Probably