Don't tell them at all.
This is what I do:
Clean the windows and be enthusiastic (you've got something better than they've ever had before). There is no reason to be negative.
1. The bristles can get into all the nooks and crannies that your fingers couldn't. Especially that tiny gap between the bottom of the window and the window sill. Use the pencil jets to blow out the dirt in that gap. If you put you're finger over one of the pencil jets, then the other jet becomes even stronger, use this method to clean out that small gap.
2. Clean the sunny side of the house first (whether it's front or back). By the time you have finished the sunny windows will have started to dry. Get the customer out of the house and show them how the windows dry lovely and clean. The top will dry first, show the custy (up close) how clean they are (and there are no marks left on the window).
3. No soap is used, so there won't be any streaks in direct sun.
4. If you really want to impress, wipe the underside of top openers (on the ground floor) and wipe the sills dry. This gives the 'impression' of less dripping water. It does work!
5. That 2 minutes spent wiping bits dry (on the first clean only) and showing the customer, up close, how clean the windows are will be time well spent.
6. If anyone asks, I always say that having wet hands, constantly, ruins the skin on your hands (completely true) and this system will help me keep my hands dry and does a great job.
7. Make sure you do a very good first clean, frames, glass, sills, doors.
A little bit of time spent on a first clean reaps benefits in the long run.