r.thin.lines: document unusual bit position

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@74215 15284696-431f-4ddb-bdfa-cd5b030d7da7

raster/r.thin/thin_lines.c

@@ -178,6 +178,12 @@ int thin_lines(int iterations)
 
 /* encode_neighbours- return neighborhood information for pixel at (middle,col) */
 
+/* bit position
+ * 1 8 7
+ * 2 x 6 
+ * 3 4 5
+ */
+
 char
 encode_neighbours(CELL * top, CELL * middle, CELL * bottom, int col, int neg)
 {