Fix BN doc

 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);

 BN_ULONG BN_get_word(BIGNUM *a);

=head1 DESCRIPTION

B<BN_ULONG> is a macro that will be an unsigned integral type optimied

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 B<BN_ULONG>.

BN_zero(), BN_one() and BN_set_word() return 1 on success, 0 otherwise.

BN_value_one() returns the constant.

=head1 BUGS

Someone might change the constant.

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.

If a B<BIGNUM> is equal to 0xffffffffL it can be represented as an

unsigned long but this value is also returned on error.

B<BN_ULONG> should probably be a typedef.

=head1 SEE ALSO