Spec orientation of each cube face, using a diagram (gpuweb#4675)

spec/index.bs

@@ -157,7 +157,7 @@ p, ul, ol, dl {
     color: var(--note-editorial-color);
 }
 
-/* Our SVGs aren't responsive to light/dark mode, so they're opaque with a
+/* Some of our SVGs aren't responsive to light/dark mode, so they're opaque with a
  * white or black background. Rounded corners make them a bit less jarring. */
 object[type="image/svg+xml"] {
     border-radius: .5em;
@@ -4292,7 +4292,22 @@ enum GPUTextureViewDimension {
     : <dfn>"cube"</dfn>
     ::
         The texture is viewed as a cubemap.
-        The view has 6 array layers, corresponding to the [+X, -X, +Y, -Y, +Z, -Z] faces of the cube.
+
+        The view has 6 array layers, each corresponding to a face of the cube in the order
+        `[+X, -X, +Y, -Y, +Z, -Z]` and the following orientations:
+
+        <figure>
+            <figcaption>
+                Cubemap faces.
+                The +U/+V axes indicate the individual faces' texture coordinates,
+                and thus the [=image copy=] memory layout of each face.
+            </figcaption>
+            <object type="image/svg+xml" data="img/cubemap.svg" width=350 height=260></object>
+        </figure>
+
+        Note: When viewed from the inside, this results in a left-handed coordinate system
+        where +X is right, +Y is up, and +Z is forward.
+
         Sampling is done seamlessly across the faces of the cubemap.
 
         Corresponding WGSL types:
@@ -4302,9 +4317,9 @@ enum GPUTextureViewDimension {
 
     : <dfn>"cube-array"</dfn>
     ::
-        The texture is viewed as a packed array of `n` cubemaps,
-        each with 6 array layers corresponding to the [+X, -X, +Y, -Y, +Z, -Z] faces of the cube.
-        Sampling is done seamlessly across the faces of the cubemaps.
+        The texture is viewed as a packed array of |n| cubemaps,
+        each with 6 array layers behaving like one {{GPUTextureViewDimension/"cube"}} view,
+        for 6|n| array layers in total.
 
         Corresponding WGSL types: