[DOC] document spread-checks

doc/configuration.txt

@@ -47,6 +47,7 @@ The following keywords are supported in the "global" section :
    - nopoll
    - nosepoll
    - tune.maxpollevents
+   - spread-checks
   
  * Debugging
    - debug
@@ -164,6 +165,13 @@ tune.maxpollevents <number>
   latency at the expense of network bandwidth, and increasing it above 200
   tends to trade latency for slightly increased bandwidth.
 
+spread-checks <0..50, in percent>
+  Sometimes it is desirable to avoid sending health checks to servers at exact
+  intervals, for instance when many logical servers are located on the same
+  physical server. With the help of this parameter, it becomes possible to add
+  some randomness in the check interval between 0 and +/- 50%. A value between
+  2 and 5 seems to show good results. The default value remains at 0.
+
 
 1.3) Debugging
 ---------------