doc: Add SSH auth callback to libssh_auth doc group

Fixes T217

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>
Reviewed-by: Jakub Jelen <jjelen@redhat.com>

include/libssh/libssh.h

@@ -637,7 +637,13 @@ LIBSSH_API ssh_pcap_file ssh_pcap_file_new(void);
 LIBSSH_API int ssh_pcap_file_open(ssh_pcap_file pcap, const char *filename);
 
 /**
- * @brief SSH authentication callback.
+ * @addtogroup libssh_auth
+ *
+ * @{
+ */
+
+/**
+ * @brief SSH authentication callback for password and publickey auth.
  *
  * @param prompt        Prompt to be displayed.
  * @param buf           Buffer to save the password. You should null-terminate it.
@@ -652,6 +658,8 @@ LIBSSH_API int ssh_pcap_file_open(ssh_pcap_file pcap, const char *filename);
 typedef int (*ssh_auth_callback) (const char *prompt, char *buf, size_t len,
     int echo, int verify, void *userdata);
 
+/** @} */
+
 LIBSSH_API ssh_key ssh_key_new(void);
 #define SSH_KEY_FREE(x) \
     do { if ((x) != NULL) { ssh_key_free(x); x = NULL; } } while(0)