user guide: fill in Default Construction Variables section

doc/user_guide.md

@@ -954,6 +954,10 @@ ${include lib/rscons/builders/disassemble.rb}
 
 ## Default Construction Variables
 
+```ruby
+${include lib/rscons/default_construction_variables.rb}
+```
+
 #> License
 
 Rscons is licensed under the terms of the MIT License:

rb/assets/user_guide.html.erb

@@ -43,6 +43,8 @@
       }
       .code {
         padding-left: 2ex;
+        width: 116ex;
+        overflow-x: auto;
       }
       .ruby_code .normal {}
       .ruby_code .comment { color: #005; font-style: italic; }