Provide some nominal documentation for `BankAccountParams`
Provide some nominal documentation for `BankAccountParams`

As mentioned in #164, it's pretty confusing that these parameters
contain both an `Account` and an `AccountID` with neither being
documented. To make matters worse, the `Account` was improperly named
and should actually be called `AccountNumber`. This patch adds a small
amount of documentation to help point out the distinction.

ChannelMeter/stripe-go,dominicphillips/stripe-go,stripe/stripe-go,froatsnook/stripe-go,mondough/stripe-go,mondough/stripe-go,froatsnook/stripe-go,dominicphillips/stripe-go,ChannelMeter/stripe-go,stripe/stripe-go