Improve Javadoc for issue 402
diff --git a/src/main/java/org/yaml/snakeyaml/scanner/Scanner.java b/src/main/java/org/yaml/snakeyaml/scanner/Scanner.java
index 2d67c26..ae195ca 100644
--- a/src/main/java/org/yaml/snakeyaml/scanner/Scanner.java
+++ b/src/main/java/org/yaml/snakeyaml/scanner/Scanner.java
@@ -56,6 +56,7 @@
      * Returns the next token.
      * <p>
      * The token will be removed from the stream.
+     * (Every invocation of this method must happen after calling checkToken())
      * </p>
      * @return the coming token
      * @throws ScannerException