Clarify the req- prefix a bit.

req- can be applied to any parameter, not just ones which are
specified as required.

bip-0021.md

@@ -33,12 +33,13 @@ Graphical bitcoin clients SHOULD register themselves as the handler for the "bit
  bitcoinurn     = "bitcoin:" bitcoinaddress [ "?" bitcoinparams ]
  bitcoinaddress = base58 *base58
  bitcoinparams  = *bitcoinparam
- bitcoinparam   = amountparam | labelparam | messageparam | otherparam | reqparam
+ standardparam  = amountparam | labelparam | messageparam | otherparam
+ bitcoinparam   = standardparam | reqparam
  amountparam    = "amount=" *digit [ "." *digit ]
  labelparam     = "label=" *pchar
  messageparam   = "message=" *pchar
  otherparam     = pchar *pchar "=" *pchar
- reqparam       = "req-" pchar *pchar "=" *pchar
+ reqparam       = "req-" standardparam
 
 === Query Keys ===