Added missing comment for method thread_group::size()

Author: HerbertKoelman

include/pthread/thread.hpp

@@ -293,6 +293,9 @@ namespace pthread {
      */
     void join();
 
+    /**
+     * @return the number of threads in the thread_group
+     */
     unsigned long size();
 
     /** return if thread_group should wait for all referenced abstract_thread terminate