Improve Vector2 / 3 / 4 normalized() classref.

Mention that the results will be unreliable with initial vector near zero.

doc/classes/Vector4.xml

@@ -191,6 +191,7 @@
 			<return type="Vector4" />
 			<description>
 				Returns the result of scaling the vector to unit length. Equivalent to [code]v / v.length()[/code]. See also [method is_normalized].
+				[b]Note:[/b] This function may return incorrect values if the input vector length is near zero.
 			</description>
 		</method>
 		<method name="posmod" qualifiers="const">