doc: clarify OPAL_RTC_READ return values
It's a quirky call. There was some discussion ages ago surrounding the
fact that we barely document what it returns and when. So, umm... make
it slightly less terrible, but still terrible.
@@ -62,6 +62,10 @@ OPAL_PARAMETER
OPAL_INTERNAL_ERROR
something went wrong, Possibly reported in error log.
+ This can be a transient error
OPAL_BUSY_EVENT
request is in flight
+
+OPAL_BUSY
+ request may be in flight
