[docs] Add a note to the StructTraits example about DataView usage.

Before this change it was somewhat easy to gloss over that the
StructTraits example requires the DataView version of the mojom type to
successfully serialize the types since it doesn't mention that in the
preamble to the example.

After this change we add a note to explicitly call this out.

Bug: N/A
Change-Id: I5784d09127968c697e0cbea0281148f4c9977389
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6346439
Commit-Queue: Justin Lulejian <jlulejian@chromium.org>
Reviewed-by: Fred Shih <ffred@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1431689}

mojo/public/cpp/bindings/README.md

@@ -1437,17 +1437,26 @@ methods:
 
 In order to define the mapping for `gfx::Rect`, we want the following
 `StructTraits` specialization, which we'll define in
-`//ui/gfx/geometry/mojo/geometry_mojom_traits.h`:
+`//ui/gfx/geometry/mojo/geometry_mojom_traits.h`.
 
-``` cpp
+*** note
+**NOTE**:  whereas in other usages of the mojom type in C++ we might directly
+use the mojom type `gfx::mojom::Rect`, our `StructTraits` template must use the
+[DataView](#using-generated-dataview-types) version (e.g.
+`gfx::mojom::RectDataView`), which exposes a direct view of the serialized Mojom
+structure within an incoming message's contents which we need in order to
+perform the serialization.
+***
+
+<pre><code>
 #include "mojo/public/cpp/bindings/mojom_traits.h"
 #include "ui/gfx/geometry/rect.h"
 #include "ui/gfx/geometry/mojo/geometry.mojom.h"
 
 namespace mojo {
 
-template <>
-class StructTraits<gfx::mojom::RectDataView, gfx::Rect> {
+template &lt;>
+class StructTraits&lt;gfx::mojom::Rect<b>DataView</b>, gfx::Rect> {
  public:
   static int32_t x(const gfx::Rect& r) { return r.x(); }
   static int32_t y(const gfx::Rect& r) { return r.y(); }
@@ -1458,7 +1467,7 @@ class StructTraits<gfx::mojom::RectDataView, gfx::Rect> {
 };
 
 }  // namespace mojo
-```
+</code></pre>
 
 And in `//ui/gfx/geometry/mojo/geometry_mojom_traits.cc`: