Before this minor change, the '--limit' and '--start-at-task' ansible-playbook command line arguments were enclosed into single quotes. Using double quotes adds a bit more flexibility, especially about the task name referred by `start_at_task` option. It also aligns with the handling of the '--extra-vars' parameter (see cb80286).
Vagrant Core Plugins
These are plugins that ship with Vagrant. Vagrant core uses its own plugin system to power a lot of the core pieces that ship with Vagrant. Each plugin will have its own README which explains its specific role.