In a way it's considered common knowledge since MilkyTracker basically copies the features of Fasttracker II, a DOS program released in the mid-90s. But we don't want to alienate newbie users either, by no means. A user manual is planned, we are very much aware that the current manual doesn't cover that area. But you could do a lot worse than checking out the old Fasttracker II manual, also available on our documentation page. It should explain the basic usage and features available in Milky as well.
Multi-sample instruments can be used to create a more authentic sound of a real-life instruments, a piano for example. Sampling multiple notes from a piano and mapping them on the instrument editor keyboard correctly creates a more natural sounding instrument than simply playing just one piano sample at different pitches. Another use is drum kits where you often use each sample at one particular pitch throughout the whole song. Grouping the whole kit under a single instrument may or may not seem logical to you, it's a personal preference if anything. Using multi-sample instruments also makes it possible to use other note scales than the typical western one.