Documentation/core-api/device_link: Add initial documentation

Document device links as introduced in v4.10 with commits:
    4bdb3550 ("driver core: Add a wrapper around
                   __device_release_driver()")
    9ed98953 ("driver core: Functional dependencies tracking
                   support")
    8c73b428 ("PM / sleep: Make async suspend/resume of devices use
                   device links")
    21d5c57b ("PM / runtime: Use device links")
    baa8809f ("PM / runtime: Optimize the use of device links")

Signed-off-by: Lukas Wunner <lukas@wunner.de>
[ jc: Moved from core-api to driver-api ]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>