Fix commit 641a5b7a14 for "nbsp" output in SVG files

In commit 641a5b7a14, 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

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;

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

@@ -72,7 +72,7 @@
 <title>a4-&gt;end</title>
 <path fill="none" stroke="#000000" d="M259,-312.5834C259,-312.5834 259,-54.659 259,-54.659"/>
 <polygon fill="#000000" stroke="#000000" points="262.5001,-54.659 259,-44.659 255.5001,-54.6591 262.5001,-54.659"/>
-<text text-anchor="middle" x="246" y="-186.6212" font-family="sans-serif" font-size="10.00" fill="#000000">true</text>
+<text text-anchor="middle" x="246" y="-186.6212" font-family="sans-serif" font-size="10.00" fill="#000000">true  </text>
 </g>
 <!-- a5 -->
 <g id="node7" class="node">
@@ -85,7 +85,7 @@
 <title>a4-&gt;a5</title>
 <path fill="none" stroke="#000000" d="M144,-298.269C144,-298.269 144,-286.5248 144,-286.5248"/>
 <polygon fill="#000000" stroke="#000000" points="147.5001,-286.5248 144,-276.5248 140.5001,-286.5249 147.5001,-286.5248"/>
-<text text-anchor="middle" x="127" y="-284.3969" font-family="sans-serif" font-size="10.00" fill="#000000">false</text>
+<text text-anchor="middle" x="127" y="-284.3969" font-family="sans-serif" font-size="10.00" fill="#000000">false   </text>
 </g>
 <!-- a6 -->
 <g id="node8" class="node">