code: func (c *Client) WalletPassphrase(passphrase string, timeoutSecs int64) error {
  	cmd := btcjson.NewWalletPassphraseCmd(passphrase, timeoutSecs)
  	_, err := c.sendCmdAndWait(cmd)
  	return err
  }
docs: // WalletPassphrase unlocks the wallet by using the passphrase to derive the
  // decryption key which is then stored in memory for the specified timeout
  // (in seconds).