Add missed description of callback function

docs/api/mg_callbacks.md

@@ -20,6 +20,8 @@
 | |The callback function `init_context()` is called after the CivetWeb server has been started and initialized, but before any requests are served. This allowes the application to perform some initialization activities before the first requests are handled.|
 |**`init_lua`**|**`void (*init_lua)( const struct mg_connection *conn, void *lua_context );`**|
 | |The callback function `init_lua()` is called just before a Lua server page is to be served. Lua page serving must have been enabled at compile time for this callback function to be called. The parameter `lua_context` is a `lua_State *` pointer.|
+|**`external_ssl_ctx`**|**`int (*external_ssl_ctx)(void **ssl_ctx, void *user_data);`**|
+| |The callback function `external_ssl_ctx()` is called when civetweb is about to create (`*ssl_ctx` is `NULL`) or free (`*ssl_ctx` is not `NULL`) a SSL context. The parameter `user_data` contains a pointer to the data which was provided to `mg_start()` when the server was started. The callback function can return 0 to signal that CivetWeb should setup the SSL context. With a return value of 1 the callback function signals CivetWeb that the SSL context has already been setup and no further processing is necessary. Also with a return value of 1 another callback function `init_ssl()` is not called. The value -1 should be returned when the SSL context initialization fails.|
 |**`init_ssl`**|**`int (*init_ssl)( void *ssl_context, void *user_data );`**|
 | |The callback function `init_ssl()` is called when CivetWeb initializes the SSL library. The parameter `user_data` contains a pointer to the data which was provided to `mg_start()` when the server was started. The callback function can return 0 to signal that CivetWeb should setup the SSL certificate. With a return value of 1 the callback function signals CivetWeb that the certificate has already been setup and no further processing is necessary. The value -1 should be returned when the SSL initialization fails.|
 |**`init_thread`**|**`void * (*init_thread)( const struct mg_context *ctx, int thread_type );`**|