Add some doc after iteration

doc/salome/gui/GEOM/input/shape_proximity.doc

@@ -3,6 +3,27 @@
 \page shape_proximity_page Shape Proximity
 
 The Shape Proximity operation calculates maximal of all possible distances between two shapes.
+Considering this case :
+
+\image html Shape_proximity_0.png
+
+The proximity of blue shape to the red one is computed like this :
+For each point of blue the distance to the red is computed using perpendicular projection. The proximity value returned is equal to maximal value of all of this distances.
+To do that the implemented algorithm
+
+1 - sampling shapes by points then calculating the distance from each sample point from one shape to another
+
+2 - find the position which gives the maximal distance
+
+3 - improve the proximity value basing on the exact shapes using the points found on step 2 as a start position
+
+It may happen that for some point of blue shape no distance to red shape exist using perpendicular projection.
+This is typically the case here :
+
+\image html Shape_proximity_1.png
+
+In the case of no perpendicular projection of a point on blue shape exists, instead of perpendicular projection the minimal distance to border point of red shape is considered.
+The distance from EndBlue Point is taken using EndRed Point (black line represents the distance for EndBlue).
 
 This is just a TUI functionality. The provided class
 <pre>