Improve some BN documentation.

Thanks to Nicolas Schodet for pointing this out.
Reviewed-by: NTim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5141)

doc/man3/BN_zero.pod

@@ -14,24 +14,26 @@ operations
 
  const BIGNUM *BN_value_one(void);
 
- int BN_set_word(BIGNUM *a, unsigned long w);
- unsigned long BN_get_word(BIGNUM *a);
+ int BN_set_word(BIGNUM *a, BN_ULONG w);
+ unsigned BN_ULONG BN_get_word(BIGNUM *a);
 
 =head1 DESCRIPTION
 
+B<BN_ULONG> is a macro that will be an unsigned integral type optimized
+for the most efficient implementation on the local platform.
+
 BN_zero(), BN_one() and BN_set_word() set B<a> to the values 0, 1 and
 B<w> respectively.  BN_zero() and BN_one() are macros.
 
 BN_value_one() returns a B<BIGNUM> constant of value 1. This constant
 is useful for use in comparisons and assignment.
 
-BN_get_word() returns B<a>, if it can be represented as an unsigned
-long.
+BN_get_word() returns B<a>, if it can be represented as a B<BN_ULONG>.
 
 =head1 RETURN VALUES
 
-BN_get_word() returns the value B<a>, and 0xffffffffL if B<a> cannot
-be represented as an unsigned long.
+BN_get_word() returns the value B<a>, or all-bits-set if B<a> cannot
+be represented as a single integer.
 
 BN_one() and BN_set_word() return 1 on success, 0 otherwise.
 BN_value_one() returns the constant.
@@ -39,8 +41,11 @@ BN_zero() never fails and returns no value.
 
 =head1 BUGS
 
-If a B<BIGNUM> is equal to 0xffffffffL it can be represented as an
-unsigned long but this value is also returned on error.
+If a B<BIGNUM> is equal to the value of all-bits-set, it will collide
+with the error condition returned by BN_get_word() which uses that
+as an error value.
+
+B<BN_ULONG> should probably be a typedef.
 
 =head1 SEE ALSO
 
@@ -53,7 +58,7 @@ versions returned an int.
 
 =head1 COPYRIGHT
 
-Copyright 2000-2017 The OpenSSL Project Authors. All Rights Reserved.
+Copyright 2000-2018 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