GUACAMOLE-547: Provide documentation for the new callback function.

src/protocols/ssh/ssh.c

@@ -137,6 +137,22 @@ static guac_common_ssh_user* guac_ssh_get_user(guac_client* client) {
 
 }
 
+/**
+ * A call-back function used to gather additional credentials from a client
+ * during a connection.  It takes the guac_client object and a string to
+ * display to the user, and returns the credentials entered by the user.
+ * 
+ * @param client
+ *     The guac_client object associated with the current connection
+ *     where additional credentials are required.
+ * 
+ * @param credName
+ *     The prompt text to display to the screen when prompting for the
+ *     additional credentials.
+ * 
+ * @return 
+ *     The string of credentials gathered from the user.
+ */
 char* guac_ssh_get_credential(guac_client *client, char* credName) {
 
     guac_ssh_client* ssh_client = (guac_ssh_client*) client->data;