Another thing to watch for an patio doors is the rubber seal that holds the glass in.
Sometimes there can be gaps in it, particularly at the top of the pane, your water will get in there and draw out any dirt or old detergent residue and can cause spotting.
More of a problem on older patio doors though.
The newer, all UPVC ones should come up tops, if you wash all of the frames too, make sure everything is thoroughly rinsed around the top of the doors.
If you are worried, then make sure it isn't the last window you clean, then when you have finished the rest you can nip back and give the glass only a light going over, making sure you don't get the frame above the glass wet, try to stay just shy of the top of the glass, you sholdn't pick up any residues this way.
Ian