Bluetooth: Add clarifying comment to l2cap_conn_ready()

There is an extra call to smp_conn_security() for outgoing LE
connections from l2cap_conn_ready() but the reason for this call is far
from clear. After a bit of commit history research and using git blame I
found out that this extra call is for socket-less pairing processes
added by commit 160dc6ac1. This patch adds a clarifying comment to the
code for this.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>