[Docs] Document the new (temp?) default of `canonifyurls = false`

[Docs] Document the new (temp?) default of `canonifyurls = false`

Also add a **Caveat** on http://gohugo.io/tutorials/github-pages-blog/
warning the reader that a `canonifyurls = true` must be added
for Spencer's old example files to work.

Fixes #802