Add overview documentation in Documentation/ABI/stable/firewire-cdev.

Improve the inline reference documentation in firewire-cdev.h:

  - Add /* available since kernel... */ comments to event numbers
    consistent with the comments on ioctl numbers.

  - Shorten some documentation on an event and an ioctl that are
    less interesting to current programming because there are newer
    preferable variants.

  - Spell Configuration ROM (name of an IEEE 1212 register) in
    upper case.

  - Move the dummy FW_CDEV_VERSION out of the reader's field of
    vision.  We should remove it from the header next year or so.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>