update doc: BN_bn2lebinpad() and BN_bn2nativepad()

Reviewed-by: NKurt Roeckx <kurt@roeckx.be>
Reviewed-by: NPaul Dale <pauli@openssl.org>
Reviewed-by: NTomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16943)

doc/man3/BN_bn2bin.pod

@@ -91,7 +91,7 @@ if B<ret> is NULL.
 BN_bn2bin() returns the length of the big-endian number placed at B<to>.
 BN_bin2bn() returns the B<BIGNUM>, NULL on error.
 
-BN_bn2binpad() returns the number of bytes written or -1 if the supplied
+BN_bn2binpad(), BN_bn2lebinpad(), and BN_bn2nativepad() return the number of bytes written or -1 if the supplied
 buffer is too small.
 
 BN_bn2hex() and BN_bn2dec() return a NUL-terminated string, or NULL