Add an example of usage to the WPACKET_reserve_bytes() documentation

 * maximum size will be. If this function is used, then it should be immediately

 * followed by a WPACKET_allocate_bytes() call before any other WPACKET

 * functions are called (unless the write to the allocated bytes is abandoned).

 * 

 * For example: If we are generating a signature, then the size of that

 * signature may not be known in advance. We can use WPACKET_reserve_bytes() to

 * handle this:

 *

 *  if (!WPACKET_sub_reserve_bytes_u16(&pkt, EVP_PKEY_size(pkey), &sigbytes1)

 *          || EVP_SignFinal(md_ctx, sigbytes1, &siglen, pkey) <= 0

 *          || !WPACKET_sub_allocate_bytes_u16(&pkt, siglen, &sigbytes2)

 *          || sigbytes1 != sigbytes2)

 *      goto err;

 */

int WPACKET_reserve_bytes(WPACKET *pkt, size_t len, unsigned char **allocbytes);