Perhaps also a comment in /etc/rc.conf or /etc/defaults/rc.conf suggesting that, if xdm is subsequently turned off manually by the user a change should also be made to /etc/ttys to restore expected behavior.
/pjr\
---- cut /etc/defaults/rc.conf ----
--- rc.conf.orig 2015-02-24 06:52:09.000000000 -0900
+++ rc.conf 2015-02-24 06:26:36.000000000 -0900
@@ -273,6 +273,8 @@
xfs=NO xfs_flags="" # X11 font server
xdm=NO xdm_flags="" # X11 display manager; needs
# wscons=YES for local displays.
+ # Also needs "console off" and
+ # "ttyE0 on"in /etc/ttys. ttys(5).
# Update fontconfig cache at boot
fccache=YES