Document the update interval tweak.

babel.man

@@ -53,7 +53,7 @@ you are doing.
 .BI \-u " update-interval"
 Specify the interval in seconds at which scheduled routing table dumps
 are made on all interfaces.  The default is either 5 times the
-wireless hello interval, the wired hello interval, or 70 seconds,
+wireless hello interval, twice wired hello interval, or 70 seconds,
 whichever is less.  Babel uses triggered updates in addition to
 scheduled updates, so this value may be set to a fairly large value.
 .TP