Improve documentation for sending body in make_requests

[jpan-box]

Hey team, Boxer here :) submitting PR for this shortly.

The helper function here:
https://github.com/box/box-python-sdk/blob/1.5/README.rst#other-requests

Demonstrates how to specify a method and URL for a custom API call. However, it is not clear how to send the body.

Looking closer at the docs:

box-python-sdk/boxsdk/client/client.py

Line 298 in f77cb41

def make_request(self, method, url, **kwargs):

It looks like it takes **kwargs, which is some sort of parameter handling. As a beginner, it's not easy to understand how to send a body for a custom API call.

See PR for suggested doc updates...