Documentation/ABI: update infiniband sysfs interfaces

Add documentation for core and hardware specific infiniband interfaces.
The descriptions have been collected from git commit logs, reading
through code and data sheets. Some drivers have incomplete doc and are
annotated with the comment '[to be documented]'.

Signed-off-by: Aishwarya Pant <aishpant@gmail.com>
Reviewed-by: Hal Rosenstock <hal@mellanox.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>