r.series.accumulate: update manual explaining accumulation

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@72228 15284696-431f-4ddb-bdfa-cd5b030d7da7

raster/r.series.accumulate/r.series.accumulate.html

@@ -3,7 +3,9 @@
 <em>r.series.accumulate</em> calculates (accumulated) raster value 
 using growing degree days (GDDs)/Winkler indices's, Biologically 
 Effective Degree Days (BEDD), Huglin heliothermal indices or an average 
-approach from several input maps.
+approach from several input maps for a given day. Accumulation of e.g. 
+degree-days to growing degree days (GDDs) can be done by providing a 
+<em>basemap</em> with GDDs of the previous day.
 <p>
 The flag <b>-a</b> determines the average computation of the input raster maps. 
 In case the flag is not set, the average calculation is: