[meta.reflection.define.aggregate] Use the usual "synopsis"-style codeblock for the class definition

Author: tkoeppe

source/meta.tex

@@ -5318,35 +5318,36 @@
 
 \rSec2[meta.reflection.define.aggregate]{Reflection class definition generation}
 
-\begin{itemdecl}
-struct data_member_options {
-  struct @\exposid{name-type}@ {                            // \expos
-    template<class T>
-      requires @\libconcept{constructible_from}@<u8string, T>
-      consteval @\exposid{name-type}@(T&&);
-
-    template<class T>
-      requires @\libconcept{constructible_from}@<string, T>
-      consteval @\exposid{name-type}@(T&&);
-
-  private:
-    variant<u8string, string> @\exposid{contents}@;         // \expos
-  };
+\indexlibraryglobal{data_member_options}%
+\begin{codeblock}
+namespace std::meta {
+  struct data_member_options {
+    struct @\exposidnc{name-type}@ {                          // \expos
+      template<class T>
+        requires @\libconcept{constructible_from}@<u8string, T>
+        consteval @\exposid{name-type}@(T&&);
+
+      template<class T>
+        requires @\libconcept{constructible_from}@<string, T>
+        consteval @\exposid{name-type}@(T&&);
+
+    private:
+      variant<u8string, string> @\exposidnc{contents}@;       // \expos
+    };
 
-  optional<@\exposid{name-type}@> name;
-  optional<int> alignment;
-  optional<int> bit_width;
-  bool no_unique_address = false;
-};
-\end{itemdecl}
+    optional<@\exposid{name-type}@> name;
+    optional<int> alignment;
+    optional<int> bit_width;
+    bool no_unique_address = false;
+  };
+}
+\end{codeblock}
 
-\begin{itemdescr}
 \pnum
 The classes \tcode{data_member_options}
 and \tcode{data_member_options::\brk{}\exposid{name-type}}
 are consteval-only types\iref{basic.types.general},
 and are not structural types\iref{temp.param}.
-\end{itemdescr}
 
 \begin{itemdecl}
 template<class T>