• Package Name: azure-storage-blob
• Package Version: 12.8.1
• Operating System: N/A
• Python Version: 3.8.6

Describe the bug
https://github.com/Azure/azure-sdk-for-python/blob/master/sdk/storage/azure-storage-blob/azure/storage/blob/_shared/policies.py#L528-L532
In the description of ExponentialRetry, it says the default behavior is "the first retry occurs after 15 seconds, the second after (15+3^1) = 18 seconds".

https://github.com/Azure/azure-sdk-for-python/blob/master/sdk/storage/azure-storage-blob/azure/storage/blob/_shared/policies.py#L565
However, the calculation of backoff time is: backoff = self.initial_backoff + (0 if settings['count'] == 0 else pow(self.increment_base, settings['count']))
And, when the first retry occurs, settings['count'] is set to 1 and the backoff would be (15+3^1) = 18 seconds, which is inconsistent with the above desctiption.

Expected behavior
Modify the description or the implementation.