consistently use "synced folders" terminology

The terminology is "synced folders" not "shared folders" (presumably to
avoid confusion with `vagrant share` which is something completely
different).
explicitly explain how to disable the default share

This caused a great deal of confusion e.g. as seen in

https://github.com/mitchellh/vagrant/issues/1004
Merge pull request #4712 from aspiers/doc-disabling-default-synced-folders

Improve docs for synced folders