Changes

Summary

1. recovered document from 0f3fdf689bf9;
2. tuned;
3. clarified parent session images, to avoid duplicate loading of theories; avoid name conflict with loaded theory src/HOL/Library/Parallel.thy;
4. clarified parent session images, to avoid duplicate loading of theories;
5. clarified meaning of "isabelle jedit -R": avoid potential problems with all_known = true;
6. clarified modules;