logging: linter nitpicks

.luacheckrc

@@ -39,6 +39,9 @@ new_read_globals = {
 	'log_fmt',
 	'log_qry',
 	'log_req',
+	'log_level',
+	'log_target',
+	'log_groups',
 	'LOG_CRIT',
 	'LOG_ERR',
 	'LOG_WARNING',

doc/config-logging-monitoring.rst

@@ -62,6 +62,7 @@ set by :func:`log_level`.
   remove all log groups, call the function with an empty table.
 
   .. code-block:: lua
+
      log_groups({'io', 'tls'}  -- turn on debug logging for io and tls groups
      log_groups()              -- list active log groups
      log_groups({})            -- remove all log groups

lib/log.c

@@ -220,8 +220,6 @@ void kr_log_level_set(kr_log_level_t level)
 	setlogmask(LOG_UPTO(kr_log_level));
 
 	kr_gnutls_log_level_set();
-
-	return;
 }
 
 void kr_log_group_add(enum kr_log_group group)