units/console-getty.service: comment reason for ConditionPathExists

Currently we have 4 getty services.  1 has a BindsTo dependency on a
device unit.  3 have ConditionPathExists, but the reason is different in
every single one.

* Add comment to [email protected] (see commit 1b41981)
* [email protected] is already commented
* container-getty.service is not strictly correct, as I realized while
  trying to compose a comment.  Reported as systemd#6584.

Author: sourcejedi

units/console-getty.service.m4.in

@@ -9,12 +9,14 @@
 Description=Console Getty
 Documentation=man:agetty(8) man:systemd-getty-generator(8)
 After=systemd-user-sessions.service plymouth-quit-wait.service
-ConditionPathExists=/dev/console
 m4_ifdef(`HAVE_SYSV_COMPAT',
 After=rc-local.service
 )m4_dnl
 Before=getty.target
 
+# OCI containers may be run without a console
+ConditionPathExists=/dev/console
+
 [Service]
 # The '-o' option value tells agetty to replace 'login' arguments with an
 # option to preserve environment (-p), followed by '--' for safety, and then