### Describe the issue

I added a second new wallet2.dat. Now I wanted to encrypt it, but `bitcoin-cli help` does not show the command to do so, I need to use `bitcoin-cli -rpcwallet=wallet2.dat help`. This is very un-intuitive, and bad for a help function.

### Can you reliably reproduce the issue?

1. have an encrypted default wallet
2. add new multi wallet wallet2.dat via config file or cmd line options
3. naive `bitcoin-cli help` does not show command to encrypt second wallet.

### Expected behaviour

All wallet encryption, lock and unlock commands should be shown unconditionally in `bitcoin-cli help`.

The dynamic hiding of some lines is arguably worse than the benefit with multiple wallets.

### Actual behaviour

`bitcoin-cli help` shows only wallet encryption commands appropriate for the default wallet.

### What version of bitcoin-core are you using?

bitcoind 0.15.0.1~dfsg-1
deb http://ppa.launchpad.net/luke-jr/bitcoin-core-bip148-unofficial-builds/ubuntu zesty main

### Machine specs:
- OS: Debian 9.2

{
    if (request.URI.substr(0, WALLET_ENDPOINT_BASE.size()) == WALLET_ENDPOINT_BASE) {
        
        std::string requestedWallet = urlDecode(request.URI.substr(WALLET_ENDPOINT_BASE.size()));
        for (CWalletRef pwallet : ::vpwallets) {
            if (pwallet->GetName() == requestedWallet) {
                return pwallet;
            }
        }
        throw JSONRPCError(RPC_WALLET_NOT_FOUND, "Requested wallet does not exist or is not loaded");
    }
    return ::vpwallets.size() == 1 || (request.fHelp && ::vpwallets.size() > 0) ? ::vpwallets[0] : nullptr;
}
std::string HelpRequiringPassphrase(CWallet * const pwallet)