[Doc] Clarify the behavior of Vector2/3.cross and mention parallel vectors

doc/classes/Vector2.xml

@@ -132,7 +132,7 @@
 			<param index="0" name="with" type="Vector2" />
 			<description>
 				Returns the 2D analog of the cross product for this vector and [param with].
-				This is the signed area of the parallelogram formed by the two vectors. If the second vector is clockwise from the first vector, then the cross product is the positive area. If counter-clockwise, the cross product is the negative area.
+				This is the signed area of the parallelogram formed by the two vectors. If the second vector is clockwise from the first vector, then the cross product is the positive area. If counter-clockwise, the cross product is the negative area. If the two vectors are parallel this returns zero, making it useful for testing if two vectors are parallel.
 				[b]Note:[/b] Cross product is not defined in 2D mathematically. This method embeds the 2D vectors in the XY plane of 3D space and uses their cross product's Z component as the analog.
 			</description>
 		</method>

doc/classes/Vector3.xml

@@ -108,6 +108,7 @@
 			<param index="0" name="with" type="Vector3" />
 			<description>
 				Returns the cross product of this vector and [param with].
+				This returns a vector perpendicular to both this and [param with], which would be the normal vector of the plane defined by the two vectors. As there are two such vectors, in opposite directions, this method returns the vector defined by a right-handed coordinate system. If the two vectors are parallel this returns an empty vector, making it useful for testing if two vectors are parallel.
 			</description>
 		</method>
 		<method name="cubic_interpolate" qualifiers="const">