Probably not the practical solution, but both.
I found writing instruction manuals you need a comprehensive manual (your b)), but to avoid a higher incidence of customers making mistakes a separate document (your a)) with just those tricky items that resulted in a high incidence of support calls helped to reduce support needs.
I don't know if it is a psychological thing with a big manual that overwhelms people, too much foreign (to them) language to wade through, people too much in a hurry to pick up a big manual but it helps having both.
Possibly the user who can use a spread sheet with some ability, knows he knows more that nothing, looks at available information.
"Calc Guide" - a large document to wade through, I am doing ok, I will read it if I have problems (but only if I realise I am having problems).
"Avoiding Gene name errors in Calc" - might be pertinent, I will check it out.
I am sure someone has done a study on the search order and persistence of people and instructions. Everyone is in a rush or think they know enough, but you never know what you don't know until it is put in front of you.
Look at a list of applicable documents, choose and open a document, check the contents page, actually get into the text of the document.
Look at your appliance instructions today, fold out step by step pictures for the impatient, a brief written manual and a downloadable comprehensive manual if required.
My Nikon came with one 500 page printed manual in the box, navigable from the contents, highly cross referenced, comprehensively indexed.
My current strategy is 1 comprehensive manual with appendices that deal with specific aspects (that cross reference to the manual) and can be printed or PDFed as stand alone documents.
I have only 1 document to maintain but print 2+ PDFs from it. The specific case appendices are highly visible in the contents page.
steve