Add comment for remaining uses of async_shutdown() why it's safe

The reason for introducing AsioTlsStream::GracefulDisconnect() was to handle
the TLS shutdown properly with a timeout since it involves a timeout. However,
the implementation of this timeout involves spwaning coroutines which are
redundant in some cases. This commit adds comments to the remaining calls of
async_shutdown() stating why calling it is safe in these places.

lib/methods/ifwapichecktask.cpp

@@ -102,6 +102,8 @@ static void DoIfwNetIo(
 	}
 
 	{
+		// Using async_shutdown() instead of AsioTlsStream::GracefulDisconnect() as this whole function
+		// is already guarded by a timeout based on the check timeout.
 		boost::system::error_code ec;
 		sslConn.async_shutdown(yc[ec]);
 	}

lib/remote/apilistener.cpp

@@ -719,6 +719,9 @@ void ApiListener::NewClientHandlerInternal(
 			// Ignore the error, but do not throw an exception being swallowed at all cost.
 			// https://github.com/Icinga/icinga2/issues/7351
 			boost::system::error_code ec;
+
+			// Using async_shutdown() instead of AsioTlsStream::GracefulDisconnect() as this whole function
+			// is already guarded by a timeout based on the connect timeout.
 			sslConn.async_shutdown(yc[ec]);
 		}
 	});