v.category: explain option=report (fixes https://trac.osgeo.org/grass/ticket/3643)

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@73331 15284696-431f-4ddb-bdfa-cd5b030d7da7

vector/v.category/main.c

@@ -668,7 +668,7 @@ int main(int argc, char *argv[])
 			    freps[i]->max[FR_ALL]);
 	    }
 	    else {
-		if (freps[i]->table != '\0') {
+		if (freps[i]->table != NULL) {
 		    fprintf(stdout, "%s: %d/%s\n", _("Layer/table"),
 			    freps[i]->field, freps[i]->table);
 		}

vector/v.category/v.category.html

@@ -37,7 +37,7 @@ is reported at the end.
     
 <p>Areas are a special case because it is impossible to attach a cat to
 an area without a centroid; in this case, the module places new
-centroids in areas automatically.
+centroids in areas automatically for <b>type=area</b>.
 
 <p>The <b>cat</b> parameter is only used with <b>option</b>=<i>add</i>, 
 <b>option</b>=<i>sum</i> and <b>option</b>=<i>del</i>.
@@ -48,6 +48,11 @@ are deleted. If <b>cat</b> is zero or positive, only this category value
 will be deleted. By default, <b>cat</b> is set to <i>1</i> which means 
 that only categories of value <i>1</i> will be deleted. 
 
+<p>With <b>option=report</b>, the module reports for each layer and 
+type the total number of categories, the minimum and the maximum 
+category number. If there are e.g. two lines with line 1 having 
+category 1 and line 1 having categories 1 and 2, then there are a total 
+of three category values with minimum 1 and maximum 2.  
 
 <p>The <b>ids</b> parameter specifies the list of feature IDs to which the
 operation is performed; by default, all vector feature ids are processed.