Rename 'optional nodes' to 'void nodes', document them, and fix some bugs

Hello @kylehardgrave, @majelbstoat, 

Please review the following commits I made in branch 'nick-void'.

dd1b9edbe0c5c606f691ae0df85f0bd243f2cb15 (2013-07-15 20:00:10 -0400)
Rename 'optional nodes' to 'void nodes', document them, and fix some bugs


R=@kylehardgrave
R=@majelbstoat