Update the documentation: G4ip/G4iSL

[hferee]

Some comments are outdated and mention non-modal propositional logic and the sequent calculus G4iP instead of G4iSL.

[samvang]

I think it would be better to also keep a reference to G4iP somewhere. I would in fact prefer it if G4iP was introduced first as a separate definition, and then G4iSL as an extension. The reason is that a potential user may very well be familiar with intuitionistic propositional logic without boxes (G4iP) and not with the version with boxes (G4iSL). If only G4iSL appears in the documentation they may miss the fact that this in particular makes it applicable to G4iP.
This remark also applies to the calculator demo.