Add missing Magick method (#9)

rmagick/rmagick#1405

Author: Watson1978

magick.html

@@ -39,6 +39,8 @@ <h3>module methods</h3>
 
         <li><a href="#limit_resource">limit_resource</a></li>
 
+        <li><a href="#set_cache_threshold">set_cache_threshold</a></li>
+
         <li><a href="#set_log_event_mask">set_log_event_mask</a></li>
 
         <li><a href="#set_log_format">set_log_format</a></li>
@@ -353,6 +355,24 @@ <h4>Notes</h4>
       </p>
     </div>
 
+    <div class="sig">
+      <h3 id="set_cache_threshold">set_cache_threshold</h3>
+
+      <p>Magick.set_cache_threshold(<span class="arg">threshold</span>)</p>
+    </div>
+
+    <div class="desc">
+      <h4>Description</h4>
+
+      <p>
+        Set the amount of free memory allocated for the pixel cache. Once this threshold is exceeded, all subsequent pixels cache operations are to/from disk.
+      </p>
+
+      <h4>Argument</h4>
+
+      <p>A number of megabytes to set.</p>
+    </div>
+
     <div class="sig">
       <h3 id="set_log_event_mask">set_log_event_mask</h3>