Index: sbin/sysctl/sysctl.8
===================================================================
--- sbin/sysctl/sysctl.8
+++ sbin/sysctl/sysctl.8
@@ -63,7 +63,10 @@
 .Fl o a
 (for compatibility).
 .It Fl a
-List all the currently available non-opaque values.
+List all the currently available values except for those which are
+opaque or excluded from listing via the
+.Dv CTLFLAG_SKIP
+flag.
 This option is ignored if one or more variable names are specified on
 the command line.
 .It Fl b