Document partial capture recommendations

[msherry]

We've seen an increase in people trying to allow for various fee + tip scenarios. This isn't documented well, so people create their own homegrown solutions, like

• creating a hold for the full amount of the transaction, voiding it, then creating a debit for full amount + tip (works, but bad user experience since the buyer sees multiple transactions)
• creating a hold, then retroactively trying to change the amount (doesn't work)

We should document the appropriate way to handle these scenarios, which is to create a hold for the full amount + some percentage, then do a partial capture to account for the amount + tip.