@@ -22,10 +22,12 @@
 
 /*!
  * @class OFCharacterSet OFCharacterSet.h ObjFW/OFCharacterSet.h
  *
  * @brief A class cluster representing a character set.
+ *
+ * @note Subclasses must implement @ref characterIsMember:.
  */
 @interface OFCharacterSet: OFObject
 #ifdef OF_HAVE_CLASS_PROPERTIES
 @property (class, readonly, nonatomic) OFCharacterSet *whitespaceCharacterSet;
 #endif