Document the BN library.

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diff --git a/doc/crypto/BN_add_word.pod b/doc/crypto/BN_add_word.pod
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+=pod
+
+=head1 NAME
+
+BN_add_word, BN_sub_word, BN_mul_word, BN_div_word, BN_mod_word - Arithmetic
+functions on BIGNUMs with integers
+
+=head1 SYNOPSIS
+
+ #include <openssl/bn.h>
+
+ int BN_add_word(BIGNUM *a, BN_ULONG w);
+
+ int BN_sub_word(BIGNUM *a, BN_ULONG w);
+
+ int BN_mul_word(BIGNUM *a, BN_ULONG w);
+
+ BN_ULONG BN_div_word(BIGNUM *a, BN_ULONG w);
+
+ BN_ULONG BN_mod_word(BIGNUM *a, BN_ULONG w);
+
+=head1 DESCRIPTION
+
+These functions perform arithmetic operations on BIGNUMs with unsigned
+integers. They are much more efficient than the normal BIGNUM
+arithmetic operations.
+
+BN_add_word() adds B<w> to B<a> (C<a+=w>).
+
+BN_sub_word() substracts B<w> from B<a> (C<a-=w>).
+
+BN_mul_word() multiplies B<a> and B<w> (C<a*=b>).
+
+BN_div_word() divides B<a> by B<w> (C<a/=w>) and returns the remainder.
+
+BN_mod_word() returns the remainder of B<a> divided by B<w> (C<a%m>).
+
+For BN_div_word() and BN_mod_word(), B<w> must not be 0.
+
+=head1 RETURN VALUES
+
+BN_add_word(), BN_sub_word() and BN_mul_word() return 1 for success, 0
+on error. The error codes can be obtained by ERR_get_error(3).
+
+BN_mod_word() and BN_div_word() return B<a>%B<w>.
+
+=head1 SEE ALSO
+
+bn(3), err(3), BN_add(3)
+
+=head1 HISTORY
+
+BN_add_word() and BN_mod_word() are available in all versions of
+SSLeay and OpenSSL. BN_div_word() was added in SSLeay 0.8, and
+BN_sub_word() and BN_mul_word() in SSLeay 0.9.0.
+
+=cut