adds guides/.document to tell rdoc not to process this directory

When the Rails 4 gem is installed rdoc sees markdown files below the
guides directory and processes them. With an empty .document file
we prevent that from happening. Thanks to @drbrain for the tip.
Combine scope conditions using AND

Currently Post.active.inactive will result in Post.inactive since
the last where clause wins when scopes are merged.

This pull request will merge all scopes ( barring defaul scope)
using AND.

The default scope will be overridden if another scope acts on the
same where clause.

closes #7365