Distinguish different compilation options in help and manpage

When moving the documentation for -ou, -fusion, and -generic_fusion from project options to main module options I noticed that there were some other things out of place. I have tried to clarify this by distinguishing three different kinds of compilation options:

• 'Compilation options' which are used whenever a module is recompiled but never trigger recompilation
• 'Project options' which are used whenever a module is recompiled and trigger recompilation of any module
• 'Main module options' which are used for the main module only and only trigger recompilation of that module

I have restructured the help and manpage to use this order, with more general options first, as well. And the 'Making a Clean project' section now explicitly refers to the different types of options.

I would expect that some options which are from my reading of the source code belong to the compilation options should actually belong to the main module options: -ci, -desc, and -exl. Should generate_code_for_project_node be changed for this?