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.

Signed-off-by: Stewart Smith <stewart@linux.ibm.com>

doc/opal-api/opal-rtc-read-3.rst

@@ -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