From the docs in the section "Patterns":
The offset can be further modified by appending a note length (see the duration chart, below). If you want to specify an offset in the middle of the first beat you can use “1.5” or “1+8”. The latter means the first beat plus the value of an eight note. This notation is quite useful when generating “swing” sequences. For example, two “swing eights” chords on beat one would be notated as: “1 81 90; 1+81 82 90”.
Just a few lines down from this the different note durations are listed. So, in your example you have "2+81" which simply means that the start point for that event is beat 2 PLUS the duration of the "first pair of a swing eights".
You are reading the section for "Bass" notes which takes liberties by not repeating the first section which gives the overall specs for the offset, duration, etc. You are also confusing "note offset" with "position". The NOTE OFFSET is the actual pitch to pick from the underlying scale for the current chord. The + and - referred to in the Bass section change the octave of the note/pitch.
Hope it's all clear now. Shout if not!