[SE-0439] Editorial clarifications on trailing comma support (#3057)

* [SE-0439] Editorial clarifications on trailing comma support

As raised in swiftlang/swift-book#421, clarify that trailing comma support is present for both generic _parameter_ lists and generic _argument_ lists.

While we're at it, add a brief sentence to the detailed design to match the decision note to clarify the requirement for a comma-separated _list_.

* [SE-0439] Fixup to use a more standard spelling for initialization

Author: xwu

proposals/0439-trailing-comma-lists.md

@@ -146,14 +146,20 @@ This proposal adds support for trailing commas in symmetrically delimited comma-
     }
     ```
 
-- Generic parameters.
+- Generic parameter lists and generic argument lists.
 
     ```swift
     struct S<
         T1,
         T2,
         T3,
     > { }
+
+    let s = S<
+        T1,
+        T2,
+        T3,
+    >()
     ```
 
 - String interpolation.
@@ -166,6 +172,8 @@ This proposal adds support for trailing commas in symmetrically delimited comma-
 
 Trailing commas will be supported in comma-separated lists when symmetric delimiters (including `(...)`, `[...]`, and `<...>`) enable unambiguous parsing.
 
+Note that the requirement for a comma-separated list means that `let x: [Int,] ❌` will not be supported, since a list of types is never supported in that context. Likewise, built-in attributes that don't parse their arguments as a comma-separated list will not support trailing comma, such as `@inline(never,) ❌`.
+
 Note that the requirement for symmetric delimiters means that the following cases will not support trailing comma:
 
 - `if`, `guard` and `while` condition lists.