Bigquery: Make consistent use of 'Optional' keyword in docstring

[HemangChothani]

Example:

python-bigquery/google/cloud/bigquery/client.py

Lines 223 to 228 in a2d5ce9

	project (str, optional):
	Project ID to use for retreiving service account email.
	Defaults to the client's project.
	retry (Optional[google.api_core.retry.Retry]): How to retry the RPC.
	timeout (Optional[float]):
	The number of seconds to wait for the underlying HTTP transport

Used multiple ways to declare a parameter as a optional.