Documentation info and "save_adv_example" only saving last example

[ramonbejar]

A little bit more of basic information explaining the settings would be useful:

• Explain what are the "unkown", "safe-incomplete" answers you get for some inputs.
• Also, give an explanation of the different settings for the "robustness_type" setting with links to the papers where they are introduced.

Also,
it seems that the setting "save_adv_example" is only saving the last counterexample found by the solver in the file "test_cex.txt". It would be more useful to have the option to save at least one counterexample for each unsafe input instance.