1: Are these drivers accompanied with proper documentation for 3rd party app developpers ?
You will need to contact the manufacturer of the adaptor to figure out the software requirements to use it in the specific platform you are interested
2: how do I find back the relevant sensor data in the captured datastream ? (after all each sensor is different)
You will need to refer to the relevant ANT+ Device Profile. You will need to sign up as an ANT+ Adopter, the device profiles are available for download in the Downloads area of the website
http://www.thisisant.com/developer/resources/downloads/#documents_tab
This resource on the Getting Started page may also be helpful
http://www.thisisant.com/developer/ant-plus/implementing-a-device-profile/
3: appart from Android, are such drivers also available for iPhone?
Please contact the manufacturer of the dongle you are interested in working with to figure out what development resources they have available.
4: The so called "plug-ins", is that just another name for driver, or is it additional aux
iliary software that sits on top of the driver and meant to access a specific sensor ?
The plugisn are specifically for Android, and they implement certain device profiles to make it simpler to communicate with sensors implementing those profiles
