Documentation for full code output

[MaxSagebaum]

In the discussion about How to print nested derivative functions (#1155) some undocumented command line parameters solved the problem. It would be good for other users if these were documented somewhere.

Possible places for the documentation:

• In README.md after Reverse mode - clad::gradient. A new section Source code output
• In README.md after Using as a plugin for Clang. A new section command line arguments
• In https://clad.readthedocs.io/en/latest/user/tutorials.html a new section Source code output
• In https://clad.readthedocs.io/en/latest/index.html a new page in the user guide command line arguments either as a new main topic or under Using clad.