v.import manual: explain snap=-1 (default setting)

scripts/v.import/v.import.html

@@ -41,7 +41,8 @@ When importing polygons, non-topological polygons are converted to
 topological areas. If the input polygons contain errors (unexpected 
 overlapping areas, small gaps between polygons, or warnings about being 
 unable to calculate centroids), the import might need to be repeated 
-using a <em>snap</em> value as suggested in the output messages.
+using a <em>snap</em> value as suggested in the output messages. The
+default value of <tt>snap=-1</tt> means that no snapping will be done.
 
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