docs: add explanation and example how 3-pass (N-pass) assemblers work and what does it mean in sjasmplus.

[ped7g]

See #179 for possible code example explaining the different passes, check also if it can be extended to other features (DEFARRAY?!).

And write some new chapter in documentation.

(if anyone has proposal how such chapter should look, or question which should be covered, please comment here)