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)

[stevexyz]

Not sure about the general case, but I think that examples often explains bettern than void descriptions.

For example I remember that the N pass solution would allow the fake instruction "jrp label", where jr would be used if label is within range jp otherwise.

[ped7g]

Partially covered by #37
Could be further extended by section focusing on source constructs which are problematic for 3-pass and how to avoid them, but maybe it would be better to just postpone this until N-pass feature is available and then document that instead. :)