Add a few documentation lines about UI_OpenSSL()

Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2204)

doc/man3/UI_new.pod

@@ -174,6 +174,9 @@ UI_get_method() returns the UI method associated with a given UI.
 
 UI_set_method() changes the UI method associated with a given UI.
 
+UI_OpenSSL() is the default OpenSSL UI method for prompting
+passphrases on the command line.
+
 =head1 COPYRIGHT
 
 Copyright 2001-2016 The OpenSSL Project Authors. All Rights Reserved.