be explicit about what's going to happen to the given columns

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@48338 15284696-431f-4ddb-bdfa-cd5b030d7da7

scripts/v.db.dropcolumn/v.db.dropcolumn.py

@@ -33,6 +33,7 @@
 #%end
 
 #%option G_OPT_DB_COLUMNS
+#% description: Name of attribute column(s) to drop
 #% required: yes
 #%end