Fixed missing code from @javax.annotation PR.

https://github.com/google/flatbuffers/pull/4986 missed part of the
implementation.

Change-Id: Ie41771eb018a550c289c77ebe9ef5c50d1ad6751

docs/source/Compiler.md

@@ -105,6 +105,8 @@ Additional options:
     output (by default the case for C++ and JS), all code will end up in
     this one file.
 
+-   `--gen-generated`: Add @Generated annotation for Java
+
 -   `--no-js-exports` :  Removes Node.js style export lines (useful for JS)
 
 -   `--goog-js-export` :  Uses goog.exportsSymbol and goog.exportsProperty