Finish period documentation, cleanup period usage.

README.md

@@ -194,17 +194,48 @@ Returns a new valid month that comes ```n``` months after ```m```. If
 This module implements simple calendar period handling with month
 granularity.
 
-sort-period-markers
+    (period-markers->periods l)
 
-period-markers->periods
+* ```l``` - list of sorted (list tag month)
 
-periods-duration
+Converts a list of period markers ```l``` into actual periods where
+each period is represented by ```(list start-month end-month)```.
 
-month-in-periods?
+The ```end-month``` may be ```#f``` in which case it is an open-ended
+period which has not ended yet.
 
-periods->string
+    (periods-duration l)
 
-periods-match
+* ```l``` - list of periods
+
+Returns the total duration in months of the periods given in the list
+```l```. Each period is represented as ```(list start-month
+end-month)```.
+
+	(month-in-periods? ps [m (*current-month*)])
+
+* ```ps``` - a list of periods
+* ```m``` - a valid month - defaults to ```(*current-month*)```
+
+Returns ```#t``` if given month ```m``` lies within any of the periods
+given in the list of periods ```ps```.
+
+    (periods->string ps)
+
+* ```ps``` - a list of periods
+
+Returns a string representing all the periods given in the list of
+periods ```ps```. The periods are represented as
+````"YYYY-MM..YYYY-MM"``` and an open end is substituded with
+```"____-__"```.
+
+
+	(periods-match ps [m (*current-month*)])
+
+* ```ps``` - a list of periods
+
+Returns the period from the list of periods ```ps``` the given month
+```m``` falls into. If no period matches, returns ```#f```.
 
 ### Primes