[saferecl.rcu.domain] Minor reogranization

In [.general, move all paragraphs below the class synopsis, add
"namespace std" to the class synopsis, and remove repeated declaration
of "rcu_default_domain".

Add a cross reference to [.func] to the header synopsis.

Author: tkoeppe

source/threads.tex

@@ -11735,6 +11735,8 @@
 
   // \ref{saferecl.rcu.domain}, class \tcode{rcu_domain}
   class rcu_domain;
+
+  // \ref{saferecl.rcu.domain.func} non-member functions
   rcu_domain& rcu_default_domain() noexcept;
   void rcu_synchronize(rcu_domain& dom = rcu_default_domain()) noexcept;
   void rcu_barrier(rcu_domain& dom = rcu_default_domain()) noexcept;
@@ -11828,31 +11830,30 @@
 
 \rSec4[saferecl.rcu.domain.general]{General}
 
+\begin{codeblock}
+namespace std {
+  class rcu_domain {
+  public:
+    rcu_domain(const rcu_domain&) = delete;
+    rcu_domain& operator=(const rcu_domain&) = delete;
+
+    void lock() noexcept;
+    bool try_lock() noexcept;
+    void unlock() noexcept;
+  };
+}
+\end{codeblock}
+
 \pnum
 This class meets the requirements of
 \oldconcept{Lockable}\iref{thread.req.lockable.req} and
 provides regions of RCU protection.
-
 \begin{example}
 \begin{codeblock}
 std::scoped_lock<rcu_domain> rlock(rcu_default_domain());
 \end{codeblock}
 \end{example}
 
-\begin{codeblock}
-class rcu_domain {
-public:
-  rcu_domain(const rcu_domain&) = delete;
-  rcu_domain& operator=(const rcu_domain&) = delete;
-
-  void lock() noexcept;
-  bool try_lock() noexcept;
-  void unlock() noexcept;
-};
-
-rcu_domain& rcu_default_domain() noexcept;
-\end{codeblock}
-
 \pnum
 The functions \tcode{lock} and \tcode{unlock} establish
 (possibly nested) regions of RCU protection.