Improves tut1.txt with more hyperlinks and minor fixes.
Improves tut2.txt with more hyperlinks.
Downgrades public module comments to private level.

This avoids the weird concatenation in the final system.html file which
doesn't make any sense.
Factors exception hierarchy into reusable fragment.

The fragment with hyperlinks is included now from the system module.
Merge pull request #1447 from gradha/pr_tutorial_improvements

Tutorial improvements