doc: clarify configuration file for Windows builds

The use of file 'config.pl' was not clearly explained.

Reported-by: liambowen@gmail.com

Discussion: https://postgr.es/m/164246013804.31952.4958087335645367498@wrigleys.postgresql.org

Backpatch-through: 10

doc/src/sgml/install-windows.sgml

@@ -128,7 +128,7 @@
  </para>
 
  <para>
-  Before you build, you may need to edit the file <filename>config.pl</filename>
+  Before you build, you can create the file <filename>config.pl</filename>
   to reflect any configuration options you want to change, or the paths to
   any third party libraries to use. The complete configuration is determined
   by first reading and parsing the file <filename>config_default.pl</filename>,