Document Tilt::Cache#fetch.

lib/tilt.rb

@@ -93,7 +93,12 @@ def initialize
       @cache = {}
     end
 
-    # @see Cache
+    # Caches a value for key, or returns the previously cached value.
+    # If a value has been previously cached for key then it is
+    # returned. Otherwise, key is yielded and the value returned by the block,
+    # which may be nil, is cached under key and returned.
+    # @yieldparam key the key passed to #fetch.
+    # @yieldreturn the value to cache for key
     def fetch(*key)
       @cache.fetch(key) do
         @cache[key] = yield