devlink: Update devlink health documentation

Update devlink-health.rst file:
- Add devlink formatted message (fmsg) API documentation.
- Add auto-dump as a condition to do dump once error reported.
- Expand OOB to clarify this acronym.

Signed-off-by: Moshe Shemesh <moshe@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>