v.rast.stats: note about vector overlap limitation (#1730)

* explain behaviour and incomplete stats if areas have multiple categories in the selected layer

Co-authored-by: Markus Metz <33666869+metzm@users.noreply.github.com>

scripts/v.rast.stats/v.rast.stats.html

@@ -31,6 +31,16 @@ in the use of the column prefix when using DBF as any additional characters
 will be chopped off.
 <p>If a MASK is present, it will be restored after the script finished.
 The script changes temporarily to the resolution of the given raster map.
+<p>If an area has several categories in the selected layer (equivalent
+to overlapping polygons in Simple Features), only one category will be
+kept during the rasterization process. Statistics for the skipped
+categories will thus be partial.
+<p>
+For example, if there are three areas: area 1 with cat 1, area 2 with
+cat 2, area 3 with cats 1, 2. Only one category value of area 3 will be
+used for rasterization, the other category value will be skipped. Thus
+statistics for the used category value will be complete, while
+statistics for the skipped category value will be incomplete.
 <p><!-- r.univar limitation -->
 Large amounts of system memory can be used when extended statistics
 (<em>first_quartile,median,third_quartile,percentile </em>) are being requested