Document the users obligation to catch when using manual transactions

[kraenhansen]

As mentioned on a comment of a different issue, when using manual transactions the user is obligated to surround any code interfacing with the Realm in a try/catch. The user also has the responsibility to react on errors/exceptions - specifically cancelling the transaction if any call to a method on the Realm throws an exception, as the state of the data cannot be guaranteed at this point.

We should add a notice about this on the online and the "in clode" documentation of the beginTransaction method.