Tenant docs update

[reshmabidikar]

Includes the following changes in the Tenants section:

1. PHP/Javascript docs for all endpoints
2. Corrections to Python/Java/Ruby code snippets as required
3. Overall corrections to endpoint descriptions/Request body/Response as required
4. Moved Tenant Key/Value section above Push Notifications section

The following is yet to be done:

1. Ruby code snippets are not added for some endpoints. The Ruby client does not include the code for these endpoints. I have filed Write missing Ruby code killbill-client-ruby#88 to track this.
2. The Javascript code snippet is not added for the Register Push Notification endpoint due to an issue in the client code. I have filed Issue in registerPushNotification killbill-client-js#146 to track this.