Update Subscription Documentation

[hitaishi9]

There should be a very clear documentation on subscription section. There are few things which needs proper clear information like:

• Auto-retry: Howmany times it will retry, can we limit that or not, will any webhook event trigger if payment gets failed
• Failed Subscription: How failed payment in subscription are managed, if auto retry happens every day at 2 AM and customer updates the payment information, what is the solution to know if thier card was successful or not. 
• Need to have a webhook for failed payment of subscription
• It should have API for suspend and re-activate the subscription
• There should be limit on auto-retry and an option to disable it. It's strange why its not there
• Card Expiring Search filter is good, it will be better if it had card expired filter