Described new cache.remove() required methods in API doc

API.html

@@ -1636,8 +1636,8 @@ <h3><a id="custom-caches" name="custom-caches">Caches</a></h3>
 </p>
 
 <p>
-A cache must provide all of these four methods: <code>lock</code>,
-<code>unlock</code>, <code>read</code>, and <code>save</code>.
+A cache must provide all of these five methods: <code>lock</code>,
+<code>unlock</code>, <code>remove</code>, <code>read</code>, and <code>save</code>.
 </p>
 
 <p>
@@ -1688,6 +1688,10 @@ <h3><a id="custom-caches" name="custom-caches">Caches</a></h3>
     # unlock a tile
     raise NotImplementedError
 
+  def remove(self, layer, coord, format):
+    # remove a tile
+    raise NotImplementedError
+
   def read(self, layer, coord, format):
     # return raw tile content from cache
     raise NotImplementedError