Swift: Add warning note to Decl.getMember.

Author: geoffw0

swift/ql/.generated.list

@@ -80,7 +80,11 @@ class Type(Element):
 @group("decl")
 class Decl(AstNode):
     module: "ModuleDecl"
-    members: list["Decl"] | child
+    members: list["Decl"] | child | desc("""
+        Prefer to use more specific methods (such as `EnumDecl.getEnumElement`) rather than relying
+        on the order of members given by `getMember`. In some cases the order of members may not
+        align with expectations, and could change in future releases.
+    """)
 
 @group("expr")
 class Expr(AstNode):