Move close_socket() out of virtual overrides section in reactor_acceptor

close_socket() is not virtual in the acceptor implementation bases
(tcp_acceptor::implementation, local_stream_acceptor::implementation).
It was misleadingly placed inside a "Virtual method overrides" comment
block. Move it out and document it as a non-virtual service helper.

Author: mvandeberg

include/boost/corosio/native/detail/reactor/reactor_acceptor.hpp

@@ -143,14 +143,11 @@ class reactor_acceptor
         return svc_;
     }
 
-    // --- Virtual method overrides ---
-
     void cancel() noexcept override { do_cancel(); }
 
+    /// Close the acceptor (non-virtual, called by the service).
     void close_socket() noexcept { do_close_socket(); }
 
-    // --- End virtual overrides ---
-
     /** Cancel a single pending operation.
 
         Claims the operation from the read_op descriptor slot