Document the UI utility functions

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

doc/man3/UI_UTIL_read_pw.pod

+=pod
+
+=head1 NAME
+
+UI_UTIL_read_pw_string, UI_UTIL_read_pw,
+*UI_UTIL_wrap_read_pem_callback - user interface utilities
+
+=head1 SYNOPSIS
+
+ #include <openssl/ui.h>
+
+ int UI_UTIL_read_pw_string(char *buf, int length, const char *prompt,
+                            int verify);
+ int UI_UTIL_read_pw(char *buf, char *buff, int size, const char *prompt,
+                     int verify);
+ UI_METHOD *UI_UTIL_wrap_read_pem_callback(pem_password_cb *cb, int
+ rwflag);
+
+=head1 DESCRIPTION
+
+UI_UTIL_read_pw_string() asks for a passphrase, using B<prompt> as a
+prompt, and stores it in B<buf>.
+The maximum allowed size is given with B<length>, including the
+terminating NUL byte.
+If B<verify> is non-zero, the password will be verified as well.
+
+UI_UTIL_read_pw() does the same as UI_UTIL_read_pw_string(), the
+difference is that you can give it an external buffer B<buff> for the
+verification passphrase.
+
+UI_UTIL_wrap_read_pem_callback() can be used to create a temporary
+B<UI_METHOD> that wraps a given PEM password callback B<cb>.
+B<rwflag> is used to specify if this method will be used for
+passphrase entry without (0) or with (1) verification.
+When not used any more, the returned method should be freed with
+UI_destroy_method().
+
+=head1 COPYRIGHT
+
+Copyright 2001-2016 The OpenSSL Project Authors. All Rights Reserved.
+
+Licensed under the OpenSSL license (the "License").  You may not use
+this file except in compliance with the License.  You can obtain a copy
+in the file LICENSE in the source distribution or at
+L<https://www.openssl.org/source/license.html>.
+
+=cut