Document new period functions.

MODULES.md

@@ -167,7 +167,21 @@ Returns a new valid month that comes ```n``` months after ```m```. If
 ### Period
 
 This module implements simple calendar period handling with month
-granularity.
+granularity. The period contains fields ```since``` which is the first
+month of the period and ```before``` which is the first month just
+after the period.
+
+    (period-since p)
+
+* ```p``` - valid period
+
+Returns the ```since``` part of given period.
+
+    (period-before p)
+
+Returns the ```since``` part of given period.
+
+Returns the ```before``` part of given period.
 
     (period-markers->periods l)