Fix commit 641a5b7 for "nbsp" output in SVG files

In commit 641a5b7, I removed "nbsp" characters from SVG files, not
realizing the SVG files were generated from GV files and that the "nbsp"
characters were caused by trailing ASCII spaces in GV files.  This
commit restores the "nbsp" SVG characters and adds a GV comment about
how the trailing spaces cause the "nbsp" output.

Reported-by: Peter Eisentraut

Discussion: https://postgr.es/m/2c5dd601-b245-4092-9c27-6d1ad51609df%40eisentraut.org

Backpatch-through: master

Author: bmomjian

doc/src/sgml/images/genetic-algorithm.gv

@@ -36,6 +36,7 @@ digraph {
 
 	// edges
 	a1 -> a2 -> a3 -> a4;
+	// these trailing spaces output as "nbsp" (UTF8 NO-BREAK SPACE)
 	a4 -> a5[xlabel="false   ", fontsize=10];
 	a4 -> end[xlabel="true  ",  fontsize=10];
 	a5 -> a6 -> a7 -> a8 -> a9;