Show mediaTypes.native.image.sizes in examples

In addition to updating the basic examples to use sizes for images and
icons, we add a section to both the API reference and the 'Show Native
Ads' page that lists both ways to define sizes for image-like assets.

Since the content is the same, it's in a shared file sourced using the
Liquid `include` keyword.

_includes/dev-docs/native-image-asset-sizes.md

@@ -0,0 +1,31 @@
+There are two methods for defining sizes for image-like assets (`image` and `icon`).  Both are shown below, but the first example (using `sizes`) is more widely supported by demand partners.
+
+1. Using `mediaTypes.native.image.sizes` (or `mediaTypes.native.icon.sizes` for icons):
+
+    ```javascript
+        mediaTypes: {
+            native: {
+                image: {
+                    required: true,
+                    sizes: [150, 50]
+                }
+            }
+        }
+    ```
+
+2. Using `mediaTypes.native.image.aspect_ratios` (or `mediaTypes.native.icon.aspect_ratios` for icons):
+
+    ```javascript
+        mediaTypes: {
+            native: {
+                image: {
+                    required: true,
+                    aspect_ratios: [{
+                        min_width: 300,        /* Optional */
+                        ratio_width: 2,        /* Required */
+                        ratio_height: 3,       /* Required */
+                 }]
+                }
+            }
+        }
+    ```

dev-docs/publisher-api-reference.md

@@ -620,15 +620,7 @@ For an example of a native ad unit, see below.  For more detailed instructions,
             native: {
                 image: {
                     required: true,
-
-                    /* If your bidder supports it, you can specify
-                       aspect ratios for image and icon native assets */
-
-                    aspect_ratios: [{
-                        min_width: 300,        /* Optional */
-                        ratio_width: 2,        /* Required */
-                        ratio_height: 3,       /* Required */
-                    }]
+                    sizes: [150, 50]
                 },
                 title: {
                     required: true,
@@ -644,7 +636,8 @@ For an example of a native ad unit, see below.  For more detailed instructions,
                     required: true
                 },
                 icon: {
-                    required: true
+                    required: true,
+                    sizes: [50, 50]
                 },
             },
             bids: [{
@@ -657,6 +650,8 @@ For an example of a native ad unit, see below.  For more detailed instructions,
     })
 ```
 
+{% include dev-docs/native-image-asset-sizes.md %}
+
 <a name="adUnit-video">
 
 ##### Video

dev-docs/show-native-ads.md

@@ -114,7 +114,8 @@ pbjs.addAdUnits({
     mediaTypes: {
         native: {
             image: {
-                required: true
+                required: true,
+                sizes: [150, 50]
             },
             title: {
                 required: true,
@@ -130,7 +131,8 @@ pbjs.addAdUnits({
                 required: true
             },
             icon: {
-                required: true
+                required: true,
+                sizes: [50, 50]
             },
         },
         bids: [{
@@ -175,15 +177,7 @@ A native "image-type" ad unit can be set up as shown in the following example.
             native: {
                 image: {
                     required: true,
-
-                    /* If your bidder supports it, you can specify aspect
-                       ratios for image and icon native assets */
-
-                    aspect_ratios: [{
-                        min_width: 300,      /* Optional */
-                        ratio_width: 2,      /* Required */
-                        ratio_height: 3,     /* Required */
-                    }]
+                    sizes: [150, 50]
                 }
             }
         }
@@ -196,6 +190,8 @@ A native "image-type" ad unit can be set up as shown in the following example.
     }];
 ```
 
+{% include dev-docs/native-image-asset-sizes.md %}
+
 ### 3. Add your native ad tag to the page body as usual:
 
 {% highlight html %}