Misleading backoff parameter

### Current Behavior

`backoff_options.retry_after` is misleading. It seems to be an absolute number of seconds. However, it's a multiplicative factor.

See `backoff_factor=self.backoff_options.retry_after,` in `PlatformClient`

And relevant implementation from urllib3:
https://github.com/urllib3/urllib3/blob/b34619f94ece0c40e691a5aaf1304953d88089de/src/urllib3/util/retry.py#L143

```
:param float backoff_factor:
A backoff factor to apply between attempts after the second try
(most errors are resolved immediately by a second try without a
delay). urllib3 will sleep for::

{backoff factor} * (2 ** ({number of previous retries}))

seconds.
```

The readme is wrong as well:

```
The exponential backoff parameters are as follows:

retry_after - The amount of time, in seconds, to wait between throttled request attempts.

Optional, will default to 5.
```

### Expected Behavior

Change the variable name to match what it does.

### Steps To Reproduce

_No response_

### Environment

```Markdown
Python Version
OS Version
...
```

### Debug logs

_No response_

### Anything else?

_No response_

Find PlatformClient and all references to backoff_options.retry_after, then compare its usage with the linked urllib3 Retry implementation. Update the parameter name and the README description so they reflect a multiplicative backoff factor, and verify that references and relevant tests remain consistent.

Written by the indexing model from the issue text.