Document config.action_dispatch.ignore_accept_header and...

Document config.action_dispatch.ignore_accept_header and config.action_dispatch.x_sendfile_header in configuring guide

guides/source/configuring.md

@@ -388,6 +388,10 @@ The schema dumper adds one additional configuration option:
 
 * `config.action_dispatch.tld_length` sets the TLD (top-level domain) length for the application. Defaults to `1`.
 
+* `config.action_dispatch.ignore_accept_header` is used to determine whether to ignore accept headers from a request. Defaults to `false`.
+
+* `config.action_dispatch.x_sendfile_header` specifies server specific X-Sendfile header. This is useful for accelerated file sending from server. For example it can be set to 'X-Sendfile' for Apache.
+
 * `config.action_dispatch.http_auth_salt` sets the HTTP Auth salt value. Defaults
 to `'http authentication'`.