Refactor figure image styles to use CSS class instead of child selector

Refactoring according the comments on https://github.com/twbs/bootstrap/pull/18104.
Closes #18131

docs/content/figures.md

@@ -6,11 +6,11 @@ group: content
 
 Anytime you need to display a piece of content—like an image—with an optional caption, consider using a `<figure>`.
 
-Use the included `.figure` and `.figure-caption` classes to provide some baseline styles for the HTML5 `<figure>` and `<figcaption>` elements. As a bonus, immediate children images are automatically responsive.
+Use the included `.figure` , `.figure-img` and `.figure-caption` classes to provide some baseline styles for the HTML5 `<figure>` and `<figcaption>` elements. Images in figures have no explicit size, so be sure to add the `.img-fluid` class to your `<img>` to make it responsive.
 
 {% example html %}
 <figure class="figure">
-  <img data-src="holder.js/400x300" class="img-rounded" alt="A generic square placeholder image with rounded corners in a figure.">
+  <img data-src="holder.js/400x300" class="figure-img img-fluid img-rounded" alt="A generic square placeholder image with rounded corners in a figure.">
   <figcaption class="figure-caption">A caption for the above image.</figcaption>
 </figure>
 {% endexample %}
@@ -19,7 +19,7 @@ Aligning the figure's caption is easy with our [text utilities]({{ site.baseurl
 
 {% example html %}
 <figure class="figure">
-  <img data-src="holder.js/400x300" class="img-rounded" alt="A generic square placeholder image with rounded corners in a figure.">
+  <img data-src="holder.js/400x300" class="figure-img img-fluid img-rounded" alt="A generic square placeholder image with rounded corners in a figure.">
   <figcaption class="figure-caption text-right">A caption for the above image.</figcaption>
 </figure>
 {% endexample %}

scss/_images.scss

@@ -40,12 +40,11 @@
 .figure {
   // Ensures the caption's text aligns with the image.
   display: inline-block;
+}
 
-  > img {
-    @extend .img-fluid;
-    margin-bottom: ($spacer-y / 2);
-    line-height: 1;
-  }
+.figure-img {
+  margin-bottom: ($spacer-y / 2);
+  line-height: 1;
 }
 
 .figure-caption {