devlink: rename devlink-info-versions.rst and add a header

Rename the devlink-info-versions.rst file to a plain devlink-info.rst
file. Add additional paragraphs explaining what devlink-info is for,
and the expectation that drivers use the generic names where plausible.

Note that drivers which use non-standard info version names ought to
document these in a driver-specific info-versions.rst file.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>