FAQ's in the manual is a good idea but one drawback will be that new data can''t be added to the manual of equipment already out the door as your FAQ data base grows.
I think it would be a good idea to have a FAQ section on your website and reference that site in the manual.
I have suggested to others who sell and maintain a site that they also put the manuals in PDF format on you website and run a e-mail news letter. If all your customers were registered to receive your newsletter, they would receive any updates, notices of new items, tips on use, etc etc.......
Finally, offer a CD for sale at a nominal fee that contains all manuals on all your equipment. I love manuals on CD. I can print a page, go do the work then just toss the page. No more hours spent looking for the manual that I put away with 50 other manuals 2 years ago.