doc: warn about makestep not working with -x option

doc/chrony.conf.adoc

@@ -1372,6 +1372,9 @@ makestep 0.1 3
 +
 This would step the system clock if the adjustment is larger than 0.1 seconds, but
 only in the first three clock updates.
++
+Note that this directive does not work and should not be used when the system
+clock control is disabled by the *-x* option of *chronyd*.
 
 [[maxchange]]*maxchange* _offset_ _start_ _ignore_::
 This directive sets the maximum offset to be accepted on a clock update. The