It also depends on the type of plane you are building.

If you're building a trainer, the manual should spell out every little step. If you're building an Aeroworks P-51, the manufacturer should be able to assume that if you're building their advanced product, you should have all building/ assembling skills down already.

Yes, there is a fine line between over-writing a manual and assuming the buyer posses the knowlege to put their plane together, but it will vary by what you're building.