[Java] Generate @return for bitset encoding of values on javadoc is a description is provided. Issue #828.

Author: mjpt777

sbe-tool/src/main/java/uk/co/real_logic/sbe/generation/java/JavaUtil.java

@@ -347,6 +347,7 @@ public static void generateOptionEncodeJavadoc(
         out.append('\n')
             .append(indent).append(" *\n")
             .append(indent).append(" * @param value true if ").append(name).append(" is set or false if not.\n")
+            .append(indent).append(" * @return this for a fluent API.\n")
             .append(indent).append(" */\n");
     }