Expand antialiasing note to clarify how to evaluate

understanding/20/contrast-enhanced.html

@@ -63,6 +63,9 @@ <h2>Intent of Contrast (Enhanced)</h2>
       </div>
 
       <div class="note">
+         <p>Because authors do not have control over user settings for font smoothing/anti-aliasing, when evaluating this
+            Success Criterion, refer to the foreground and background colors obtained from the user agent, or the underlying
+            markup and stylesheets, rather than the text as presented on screen.</p>
          <p>Due to anti-aliasing, particularly thin or unusual fonts may be rendered by user agents with a much fainter
             color than the actual text color defined in the underlying CSS. This can lead to situations where text has
             a contrast ratio that nominally passes the Success Criterion, but has a much lower contrast in practice.

understanding/20/contrast-minimum.html

@@ -63,6 +63,9 @@ <h2>Intent of Contrast (Minimum)</h2>
       </div>
 
       <div class="note">
+         <p>Because authors do not have control over user settings for font smoothing/anti-aliasing, when evaluating this
+            Success Criterion, refer to the foreground and background colors obtained from the user agent, or the underlying
+            markup and stylesheets, rather than the text as presented on screen.</p>
          <p>Due to anti-aliasing, particularly thin or unusual fonts may be rendered by user agents with a much fainter
             color than the actual text color defined in the underlying CSS. This can lead to situations where text has
             a contrast ratio that nominally passes the Success Criterion, but has a much lower contrast in practice.