Update documentation for sprite and image widgets

Added emphasis on setting width and height for sprite and image widgets.

wgui/doc/widgets.md

@@ -160,7 +160,9 @@ _2nd gradient color_
 
 ### `<sprite>`
 
-### Image widget for small images,
+### Image widget for small images
+
+Always set the `width` & `height` of a <sprite>!
 
 Supported formats: svg, png, jpeg, gif, webp
 
@@ -196,6 +198,8 @@ _wgui internal image path. Do not use directly unless it's related to the core w
 
 ### Image widget for large images
 
+Always set the `width` & `height` of an <image>!
+
 Supported formats: svg, png, jpeg, gif, webp
 
 Maximum image size: Max texture size for the GPU (usually 8K+)