The command to generate the cap db when login.conf.d is in use isn't
immediately obvious as login.conf.d takes precedence which then
necessitates the use of -f. Add example to login.conf.5 matching the
example without login.conf.d. Command courtesy of Solène.

diff --git login.conf.5 login.conf.5
index 3d6d92a14c3..66fa3f543e5 100644
--- login.conf.5
+++ login.conf.5
@@ -81,6 +81,12 @@ the following command may be used:
 .Pp
 .Dl # cap_mkdb /etc/login.conf
 .Pp
+Or if
+.Pa /etc/login.conf.d
+is in use:
+.Pp
+.Dl # cap_mkdb -f /etc/login.conf /etc/login.conf.d/* /etc/login.conf
+.Pp
 Note that
 .Xr cap_mkdb 1
 must be run after each edit of