docs: add intro text to structured output match behavior table

Author: jpr5

docs/structured-output/index.html

@@ -129,6 +129,12 @@ <h2>JSON Fixture</h2>
         </div>
 
         <h2>Match Behavior</h2>
+        <p>
+          Each row shows whether a request matches a fixture based on the combination of their
+          <code>responseFormat</code> values. A fixture without <code>responseFormat</code> is a
+          wildcard (matches all requests), but a fixture with <code>responseFormat</code> is a
+          filter (only matches requests that specify that format).
+        </p>
         <table class="endpoint-table">
           <thead>
             <tr>