doc: rewrite stale evalComptimeString comment (no shortcut still exists)

src/ir/lower.zig

@@ -6442,12 +6442,14 @@ pub const Lowering = struct {
         }
 
         // Case 2: Evaluate via IR interpreter, reusing the parent module.
-        // The parent module already has every protocol/struct/impl/thunk
-        // registered (Allocator, CAllocator, Context, the GPA/Tracker
-        // thunks), so the interp can run the full protocol-dispatch
-        // chain that codegen emits. A fresh ct_module would skip the
-        // scan pass and force every `context.allocator.X` call through
-        // a `matchContextAllocCall` shortcut to stay runnable.
+        // The parent's `scanDecls` pass has already registered every
+        // type / protocol / impl / thunk the comptime call may need
+        // (Allocator, CAllocator, Context, the per-impl thunks). A
+        // fresh empty module would only lazy-lower function ASTs and
+        // would miss the type/protocol registrations, which would break
+        // `context.allocator.X` — the protocol dispatch chain needs
+        // those types to resolve struct field layout and the alloc/
+        // dealloc thunks at the bottom of the dispatch.
         const ct_func_id = self.createComptimeFunction("__insert", expr, .string);
 
         var interp = interp_mod.Interpreter.init(self.module, self.alloc);