removed info about deprecated rake tasks, documented the sandbox option for the rails console and minor corrections

Verify that creating a has_many through record where there is a default_scope on the join model works correctly (creates the join record with the default scope applied)

Dont set explicit domain for cookies if host is not a domain name

[#6002 state:committed]
Signed-off-by: NSantiago Pastorino <santiago@wyeworks.com>

When using stylesheet_link_tag(:all) or javascript_include_tag(:all),
assets directories are searched for css or js files. When
config.serve_static_assets is set to true, those files can be
served directly from engine's directories. That's why assets paths
should be set individually for controllers inside engine
if we want to serve static assets with ActionDispatch::Static