devlink: convert devlink-health.txt to rst format

Update the devlink-health documentation to use the newer
ReStructuredText format.

Note that it's unclear what OOB stood for, and it has been left as-is
without a proper first-use expansion of the acronym.

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