callbacks: Improve documentation about callbacks handling

Fixes T103

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Anderson Toshiyuki Sasaki <ansasaki@redhat.com>

include/libssh/callbacks.h

@@ -354,6 +354,9 @@ typedef struct ssh_server_callbacks_struct *ssh_server_callbacks;
  * This functions sets the callback structure to use your own callback
  * functions for user authentication, new channels and requests.
  *
+ * Note, that the structure is not copied to the session structure so it needs
+ * to be valid for the whole session lifetime.
+ *
  * @code
  * struct ssh_server_callbacks_struct cb = {
  *   .userdata = data,
@@ -548,6 +551,9 @@ typedef struct ssh_packet_callbacks_struct *ssh_packet_callbacks;
  * This functions sets the callback structure to use your own callback
  * functions for auth, logging and status.
  *
+ * Note, that the callback structure is not copied into the session so it needs
+ * to be valid for the whole session lifetime.
+ *
  * @code
  * struct ssh_callbacks_struct cb = {
  *   .userdata = data,
@@ -849,7 +855,11 @@ typedef struct ssh_channel_callbacks_struct *ssh_channel_callbacks;
  * @brief Set the channel callback functions.
  *
  * This functions sets the callback structure to use your own callback
- * functions for channel data and exceptions
+ * functions for channel data and exceptions.
+ *
+ * Note, that the structure is not copied to the channel structure so it needs
+ * to be valid as for the whole life of the channel or until it is removed with
+ * ssh_remove_channel_callbacks().
  *
  * @code
  * struct ssh_channel_callbacks_struct cb = {