v.extract: update manual, mention layer=-1

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@72670 15284696-431f-4ddb-bdfa-cd5b030d7da7

vector/v.extract/v.extract.html

@@ -14,10 +14,12 @@ extracted. Categories are not changed in that case.
 
 <h2>NOTES</h2>
 
-Only features with a category number will be extracted. So if you want to 
-extract boundaries (which are usually without category, as that information
-is normally held in the area's centroid) you must first use 
-<em><a href="v.category.html">v.category</a></em> to add them.
+Only features with a category number in the selected layer will be 
+extracted. So if you want to extract boundaries (which are usually 
+without category, as that information is normally held in the area's 
+centroid) you must first use 
+<em><a href="v.category.html">v.category</a></em> to add them, or use 
+<b>layer=-1</b>.
 
 <h2>EXAMPLES</h2>