Parser / PreParser unification: Add docs.

R=rossberg@chromium.org, rossberg
BUG=v8:3126
LOG=N

Review URL: https://codereview.chromium.org/196953004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19861 ce2b1a6d-e550-0410-aec6-3dcde31c8c00