Clarify units for accepted_time_diff config param

The `accepted_time_diff` param is taken as seconds diff, but this isn't clear from the docs without digging in the source code.

Author: tbrisker-april

docs/howto/config.rst

@@ -526,7 +526,7 @@ accepted_time_diff
 
 If your computer and another computer that you are communicating with are not
 in sync regarding the computer clock, then here you can state how big a
-difference you are prepared to accept.
+difference in seconds you are prepared to accept.
 
 .. note:: This will indiscriminately affect all time comparisons.
     Hence your server may accept a statement that in fact is too old.