Add method description to Vector3i Class Reference

Add description for the abs method in the Vector3i class. Description added is identical to the abs method for the other vector classes

doc/classes/Vector3i.xml

@@ -48,6 +48,7 @@
 		<method name="abs" qualifiers="const">
 			<return type="Vector3i" />
 			<description>
+				Returns a new vector with all components in absolute values (i.e. positive).
 			</description>
 		</method>
 		<method name="clamp" qualifiers="const">