Make it clear which client methods do not make an API call

In both the README and in client.py, it isn't made clear that `Client.folder()`, `Client.file()`, etc. do not make an API call by default (they initialize an empty object, which can then be used to make API calls).

We should call this out, both in the README, and in the docstrings of those methods.

Start by reading the client methods in client.py, including Client.folder() and Client.file(), then check the corresponding README usage or API documentation. Update the README and method docstrings so they clearly state that these methods initialize empty objects without making an API call by default; done means both locations communicate this behavior.

索引モデルが issue の本文から書いたものです。