Material: document disabling depth test disables depth write caveat. (#…

…27000)

When setting the `depthTest` flag of a material to false, this will also implicitly disable the depth write, because `glDisable(GL_DEPTH_TEST)` is called internally. 

See: https://www.khronos.org/opengl/wiki/Depth_Test
> When depth testing is disabled, writes to the depth buffer are also disabled. 

While the behavior makes sense I think (`depthFunc` can be used to achieve the desired behavior), it caught me off guard, so I thought it would make sense to document it.

docs/api/en/materials/Material.html

@@ -161,7 +161,8 @@ <h3>[property:Integer depthFunc]</h3>
 		<h3>[property:Boolean depthTest]</h3>
 		<p>
 			Whether to have depth test enabled when rendering this material. Default
-			is `true`.
+			is `true`. When the depth test is disabled, the depth write will also be
+			implicitly disabled.
 		</p>
 
 		<h3>[property:Boolean depthWrite]</h3>