Add documentation for BN_with_flags

Following on from the previous commit this adds some documentation for the
BN_with_flags function which is easy to misuse.
Reviewed-by: NEmilia Käsper <emilia@openssl.org>

doc/crypto/BN_copy.pod

@@ -2,7 +2,7 @@
 
 =head1 NAME
 
-BN_copy, BN_dup - copy BIGNUMs
+BN_copy, BN_dup, BN_with_flags - copy BIGNUMs
 
 =head1 SYNOPSIS
 
@@ -12,11 +12,41 @@ BN_copy, BN_dup - copy BIGNUMs
 
  BIGNUM *BN_dup(const BIGNUM *from);
 
+ void BN_with_flags(BIGNUM *dest, const BIGNUM *b, int flags);
+
 =head1 DESCRIPTION
 
 BN_copy() copies B<from> to B<to>. BN_dup() creates a new B<BIGNUM>
 containing the value B<from>.
 
+BN_with_flags creates a B<temporary> shallow copy of B<b> in B<dest>. It places
+significant restrictions on the copied data. Applications that do no adhere to
+these restrictions may encounter unexpected side effects or crashes. For that
+reason use of this function is discouraged. Any flags provided in B<flags> will
+be set in B<dest> in addition to any flags already set in B<b>. For example this
+might commonly be used to create a temporary copy of a BIGNUM with the
+B<BN_FLG_CONSTTIME> flag set for constant time operations. The temporary copy in
+B<dest> will share some internal state with B<b>. For this reason the following
+restrictions apply to the use of B<dest>:
+
+=over 4
+
+=item *
+
+B<dest> should be a newly allocated BIGNUM obtained via a call to BN_new(). It
+should not have been used for other purposes or initialised in any way.
+
+=item *
+
+B<dest> must only be used in "read-only" operations, i.e. typically those
+functions where the relevant parameter is declared "const".
+
+=item *
+
+B<dest> must be used and freed before any further subsequent use of B<b>
+
+=back
+
 =head1 RETURN VALUES
 
 BN_copy() returns B<to> on success, NULL on error. BN_dup() returns

include/openssl/bn.h

@@ -285,9 +285,11 @@ int BN_get_flags(const BIGNUM *b, int n);
 
 /*
  * get a clone of a BIGNUM with changed flags, for *temporary* use only (the
- * two BIGNUMs cannot not be used in parallel!)
+ * two BIGNUMs cannot be used in parallel!). Also only for *read only* use. The
+ * value |dest| should be a newly allocated BIGNUM obtained via BN_new() that
+ * has not been otherwise initialised or used.
  */
-void BN_with_flags(BIGNUM *dest, const BIGNUM *b, int n);
+void BN_with_flags(BIGNUM *dest, const BIGNUM *b, int flags);
 
 /* Wrapper function to make using BN_GENCB easier,  */
 int BN_GENCB_call(BN_GENCB *cb, int a, int b);