Update E0562 to account for the new impl trait positions

compiler/rustc_error_codes/src/error_codes/E0562.md

@@ -1,5 +1,4 @@
-Abstract return types (written `impl Trait` for some trait `Trait`) are only
-allowed as function and inherent impl return types.
+`impl Trait` is only allowed as a function return and argument type.
 
 Erroneous code example:
 
@@ -14,7 +13,7 @@ fn main() {
 }
 ```
 
-Make sure `impl Trait` only appears in return-type position.
+Make sure `impl Trait` appears in a function signature.
 
 ```
 fn count_to_n(n: usize) -> impl Iterator<Item=usize> {
@@ -28,6 +27,6 @@ fn main() {
 }
 ```
 
-See [RFC 1522] for more details.
+See the [reference] for more details on `impl Trait`.
 
-[RFC 1522]: https://github.com/rust-lang/rfcs/blob/master/text/1522-conservative-impl-trait.md
+[reference]: https://doc.rust-lang.org/stable/reference/types/impl-trait.html