knot.conf manual: add system.background workers

man/knot.conf.5.in

@@ -48,6 +48,12 @@ serves as an example of the configuration for knotc(8) and knotd(8).
   # Default: unset (auto-estimates optimal value from the number of online CPUs)
   # workers 3;
 
+  # Number of background workers
+  # This option is used to set number of threads used to execute background
+  # operations (e.g., zone loading, zone signing, XFR zone updates, ...)
+  # Default: unset (auto-estimates optimal value from the number of online CPUs)
+  # background-workers 4;
+
   # User for running server
   # May also specify user.group (e.g. knot.users)
   # user knot.users;