Review feedback from @dangell7 and @gregtatcam

- Mostly adding comments.
- Fixed some function parameter names that weren't updated after
  a copy/paste.
- LoanBrokerCoverWithdraw does not need to check for freeze when sending
  to the issuer.
- Reorder the fund transfer cases in LoanBrokerCoverWithdraw to make it
  clearer that some transfers are direct, and some use the payment
  engine.
- Only look up the vault ID once in LoanBrokerSet

include/xrpl/protocol/Indexes.h

@@ -348,18 +348,18 @@ Keylet
 loanbroker(AccountID const& owner, std::uint32_t seq) noexcept;
 
 inline Keylet
-loanbroker(uint256 const& vaultKey)
+loanbroker(uint256 const& key)
 {
-    return {ltLOAN_BROKER, vaultKey};
+    return {ltLOAN_BROKER, key};
 }
 
 Keylet
 loan(uint256 const& loanBrokerID, std::uint32_t loanSeq) noexcept;
 
 inline Keylet
-loan(uint256 const& vaultKey)
+loan(uint256 const& key)
 {
-    return {ltLOAN, vaultKey};
+    return {ltLOAN, key};
 }
 
 Keylet