Document the Introduced struct

Co-authored-by: Dimi Racordon <[email protected]>

Sources/Core/AST/Introduced.swift

@@ -1,10 +1,16 @@
 /// A node and the site of its introducer in program sources.
 public struct Introduced<T: NodeIDProtocol>: Codable {
+
+  /// The site of `value`'s introducer.
   public let introducerSite: SourceRange
+
+  /// A node introduced in program sources at `introducerSite`.
   public let value: T
 
-  public init(introducerSite: SourceRange, value: T) {
+  /// Creates an instance bundling `n` with the site from which its introducer was parsed.
+  public init(_ n: T, at introducerSite: SourceRange) {
     self.introducerSite = introducerSite
-    self.value = value
+    self.value = n
   }
+
 }