Edit to two figure captions in Understanding Focus Appearance (#4925)

I copied a few words from the alt text into the `figcaption` as
parenthetical for figures 18 and 19 because, when read in isolation, it
wasn’t visually obvious what was the problem. I also added a bit of
emphasis on key words.

This follows follows from something I notice when reviewing #4890 


[Preview](https://deploy-preview-4925--wcag2.netlify.app/understanding/focus-appearance#relationship-with-non-text-contrast)
and
[diff](https://services.w3.org/htmldiff?doc1=https%3A%2F%2Fwww.w3.org%2FWAI%2FWCAG22%2FUnderstanding%2Ffocus-appearance&doc2=https%3A%2F%2Fdeploy-preview-4925--wcag2.netlify.app%2Funderstanding%2Ffocus-appearance#relationship-with-non-text-contrast)
anchored to “Relationship with non-text Contrast” heading, about
two-thirds through the page.

Co-authored-by: Patrick H. Lauke <redux@splintered.co.uk>

Author: bruce-usab

understanding/22/focus-appearance.html

@@ -441,16 +441,16 @@ <h3>Relationship with Non-text Contrast</h3>
 
                 <img src="img/focus-indicator-good-change-bad-adjacent.png"
                     alt="A light blue button with a 3px thick border. The border is black in the unfocused state and light gray in the focused state.">
-                <figcaption>This example passes Focus Appearance but fails Non-text Contrast; there is insufficient
-                    <em>adjacent contrast</em> between the focus indicator and the adjacent colors.
+                <figcaption>This example passes Focus Appearance but <em>fails Non-text Contrast</em>; there is insufficient
+                    <strong>adjacent contrast</strong> between the focus indicator (light gray border) and the adjacent colors (white background and light blue button).
                 </figcaption>
             </figure>
 
             <figure id="non-text-contrast-pass-focus-appearance-fail">
                 <img src="img/focus-indicator-good-adjacent-bad-change.png"
                     alt="A light blue button with a 3px thick border. The border is black in the unfocused state and dark gray in the focused state.">
-                <figcaption>This example passes Non-text Contrast but fails Focus Appearance; there is insufficient
-                    <em>change of contrast</em> between the focused and unfocused states.
+                <figcaption>This example passes Non-text Contrast but <em>fails Focus Appearance</em>; there is insufficient
+                    <strong>change of contrast</strong> between the focused (dark gray border) and unfocused states (black border).
                 </figcaption>
 
             </figure>
@@ -744,4 +744,4 @@ <h2>Related Resources</h2>
     {% include "understanding/techniques.html" %}
 </body>
 
-</html>
+</html>