libssh2_session_callback_set: extended the man page

docs/libssh2_session_callback_set.3

@@ -4,12 +4,16 @@
 .SH NAME
 libssh2_session_callback_set - set a callback function
 .SH SYNOPSIS
+.nf
 #include <libssh2.h>
 
-void *
-libssh2_session_callback_set(LIBSSH2_SESSION *session, int cbtype, void *callback);
-
+void *libssh2_session_callback_set(LIBSSH2_SESSION *session,
+                                   int cbtype, void *callback);
 .SH DESCRIPTION
+Sets a custom callback handler for a previously initialized session
+object. Callbacks are triggered by the receipt of special packets at the
+Transport layer. To disable a callback, set it to NULL.
+
 \fIsession\fP - Session instance as returned by 
 .BR libssh2_session_init_ex(3)
 
@@ -17,14 +21,19 @@ libssh2_session_callback_set(LIBSSH2_SESSION *session, int cbtype, void *callbac
 
 \fIcallback\fP - Pointer to custom callback function. The prototype for 
 this function must match the associated callback declaration macro.
-
-Sets a custom callback handler for a previously initialized session 
-object. Callbacks are triggered by the receipt of special packets at 
-the Transport layer. To disable a callback, set it to NULL.
-
+.SH CALLBACK TYPES
+.IP LIBSSH2_CALLBACK_IGNORE
+Called when a SSH_MSG_IGNORE message is received
+.IP LIBSSH2_CALLBACK_DEBUG
+Called when a SSH_MSG_DEBUG message is received
+.IP LIBSSH2_CALLBACK_DISCONNECT
+Called when a SSH_MSG_DISCONNECT message is received
+.IP LIBSSH2_CALLBACK_MACERROR
+Called when a mismatched MAC has been detected in the transport layer
+.IP LIBSSH2_CALLBACK_X11
+Called when an X11 connection has been accepted
 .SH RETURN VALUE
-Pointer to previous callback handler. Returns NULL if no 
-prior callback handler was set.
-
+Pointer to previous callback handler. Returns NULL if no prior callback
+handler was set or the callback type was unknown.
 .SH SEE ALSO
 .BR libssh2_session_init_ex(3)