Add comment to keep similar instructions in sync

wiki/src/doc/advanced_topics/boot_options.mdwn

@@ -42,6 +42,8 @@ adding a boot option [[using GRUB|boot_options#grub]] below.</p>
 
 To add a boot option in *SYSLINUX*:
 
+<!-- Keep in sync with install/inc/steps/restart_first_time.inline.mdwn. -->
+
 1. Press **Tab** when *SYSLINUX* appears.
 
    A list of boot options
@@ -77,6 +79,8 @@ adding a boot option [[using SYSLINUX|boot_options#syslinux]] above.</p>
 
 To add a boot option in *GRUB*:
 
+<!-- Keep in sync with install/inc/steps/restart_first_time.inline.mdwn. -->
+
 1. Press **e** when GRUB appears.
 
    A new screen appears with more options.