tracepoints: API doc update to tracepoint_probe_register() return value

Describe the return values of tracepoint_probe_register(), including
-ENODEV added by commit:

Author: Steven Rostedt <rostedt@goodmis.org>

    tracing: Warn if a tracepoint is not set via debugfs

Link: http://lkml.kernel.org/r/1394499898-1537-2-git-send-email-mathieu.desnoyers@efficios.com

CC: Ingo Molnar <mingo@kernel.org>
CC: Frederic Weisbecker <fweisbec@gmail.com>
CC: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/tracepoint.c

@@ -378,7 +378,17 @@ tracepoint_add_probe(const char *name, void *probe, void *data)
  * @probe: probe handler
  * @data: probe private data
  *
- * Returns 0 if ok, error value on error.
+ * Returns:
+ * - 0 if the probe was successfully registered, and tracepoint
+ *   callsites are currently loaded for that probe,
+ * - -ENODEV if the probe was successfully registered, but no tracepoint
+ *   callsite is currently loaded for that probe,
+ * - other negative error value on error.
+ *
+ * When tracepoint_probe_register() returns either 0 or -ENODEV,
+ * parameters @name, @probe, and @data may be used by the tracepoint
+ * infrastructure until the probe is unregistered.
+ *
  * The probe address must at least be aligned on the architecture pointer size.
  */
 int tracepoint_probe_register(const char *name, void *probe, void *data)