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>

Documentation/networking/devlink/devlink-health.rst

@@ -33,7 +33,7 @@ Device driver can provide specific callbacks for each "health reporter", e.g.:
   * Recovery procedures
   * Diagnostics procedures
   * Object dump procedures
-  * OOB initial parameters
+  * Out Of Box initial parameters
 
 Different parts of the driver can register different types of health reporters
 with different handlers.
@@ -46,12 +46,31 @@ Once an error is reported, devlink health will perform the following actions:
   * A log is being send to the kernel trace events buffer
   * Health status and statistics are being updated for the reporter instance
   * Object dump is being taken and saved at the reporter instance (as long as
-    there is no other dump which is already stored)
+    auto-dump is set and there is no other dump which is already stored)
   * Auto recovery attempt is being done. Depends on:
 
     - Auto-recovery configuration
     - Grace period vs. time passed since last recover
 
+Devlink formatted message
+=========================
+
+To handle devlink health diagnose and health dump requests, devlink creates a
+formatted message structure ``devlink_fmsg`` and send it to the driver's callback
+to fill the data in using the devlink fmsg API.
+
+Devlink fmsg is a mechanism to pass descriptors between drivers and devlink, in
+json-like format. The API allows the driver to add nested attributes such as
+object, object pair and value array, in addition to attributes such as name and
+value.
+
+Driver should use this API to fill the fmsg context in a format which will be
+translated by the devlink to the netlink message later. When it needs to send
+the data using SKBs to the netlink layer, it fragments the data between
+different SKBs. In order to do this fragmentation, it uses virtual nests
+attributes, to avoid actual nesting use which cannot be divided between
+different SKBs.
+
 User Interface
 ==============