Clean top half of doors down to railings as normal. Rinse.
Angle head of brush at a shallow angle to the pole.
Hold pole so brush is near vertical and place in between bars so that when you turn brush horizontal it's edge just covers the edge of the side-frame. Scrub up and down, pressing in tight and going as high as possible up under the top cross bar of the railings and then down to the bottom frame.
Turn pole so that brush (still between the same bars) now extends as far as possible to the other side. Repeat above. Turn brush as vertical as poss. and remove from bars.
Move across and repeat as above so that you are putting the brush in between only the railings that are necessary to scrub all the glass.
Rest back of brush or top of pole on top railing and run from side to side rinsing the glass from this half way point.
It's a knack you'll get for each job.
Oh! Make sure your brush is tight so that it doesn't swivel and get trapped between the bars! And this work is a nightmare with an autobrush (the top bar fouls the bars too easily.)
Modified to add:- Dai beat me to it! 