Document how Vector2, Vector3 and Color behave in a boolean context

See #39731.

(cherry picked from commit 227f12478d)

doc/classes/Color.xml

@@ -7,6 +7,7 @@
 		A color is represented by red, green, and blue [code](r, g, b)[/code] components. Additionally, [code]a[/code] represents the alpha component, often used for transparency. Values are in floating-point and usually range from 0 to 1. Some properties (such as [member CanvasItem.modulate]) may accept values greater than 1.
 		You can also create a color from standardized color names by using [method @GDScript.ColorN] or directly using the color constants defined here. The standardized color set is based on the [url=https://en.wikipedia.org/wiki/X11_color_names]X11 color names[/url].
 		If you want to supply values in a range of 0 to 255, you should use [method @GDScript.Color8].
+		[b]Note:[/b] In a boolean context, a Color will evaluate to [code]false[/code] if it's equal to [code]Color(0, 0, 0, 1)[/code] (opaque black). Otherwise, a Color will always evaluate to [code]true[/code].
 	</description>
 	<tutorials>
 	</tutorials>

doc/classes/Vector2.xml

@@ -5,6 +5,7 @@
 	</brief_description>
 	<description>
 		2-element structure that can be used to represent positions in 2D space or any other pair of numeric values.
+		[b]Note:[/b] In a boolean context, a Vector2 will evaluate to [code]false[/code] if it's equal to [code]Vector2(0, 0)[/code]. Otherwise, a Vector2 will always evaluate to [code]true[/code].
 	</description>
 	<tutorials>
 		<link>https://docs.godotengine.org/en/latest/tutorials/math/index.html</link>

doc/classes/Vector3.xml

@@ -5,6 +5,7 @@
 	</brief_description>
 	<description>
 		3-element structure that can be used to represent positions in 3D space or any other pair of numeric values.
+		[b]Note:[/b] In a boolean context, a Vector3 will evaluate to [code]false[/code] if it's equal to [code]Vector3(0, 0, 0)[/code]. Otherwise, a Vector3 will always evaluate to [code]true[/code].
 	</description>
 	<tutorials>
 		<link>https://docs.godotengine.org/en/latest/tutorials/math/index.html</link>