### 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

{
    LOCK(pWallet->cs_wallet);
    pWallet->nRelockTime = 0;
    pWallet->Lock();
}
UniValue walletpassphrase(const JSONRPCRequest& request)
{
    CWallet * const pwallet = GetWalletForJSONRPCRequest(request);
    if (!EnsureWalletIsAvailable(pwallet, request.fHelp)) {
        return NullUniValue;
    }
    if (pwallet->IsCrypted() && (request.fHelp || request.params.size() != 2)) {
        throw std::runtime_error(
            "walletpassphrase \"passphrase\" timeout\n"
            "\nStores the wallet decryption key in memory for 'timeout' seconds.\n"
            "This is needed prior to performing transactions related to private keys such as sending bitcoins\n"
            "\nArguments:\n"
            "1. \"passphrase\"     (string, required) The wallet passphrase\n"
            "2. timeout            (numeric, required) The time to keep the decryption key in seconds.\n"
            "\nNote:\n"
            "Issuing the walletpassphrase command while the wallet is already unlocked will set a new unlock\n"
            "time that overrides the old one.\n"
            "\nExamples:\n"
            "\nUnlock the wallet for 60 seconds\n"
            + HelpExampleCli("walletpassphrase", "\"my pass phrase\" 60") +
            "\nLock the wallet again (before 60 seconds)\n"
            + HelpExampleCli("walletlock", "") +
            "\nAs json rpc call\n"
            + HelpExampleRpc("walletpassphrase", "\"my pass phrase\", 60")
        );