[Request] Verify User Project methods comments are not ambiguous

[frankyn]

General Request:

Verify that the comments are clear for methods which accept User Project with and without Requester Pays.

Here's a distinction between Requester Pays and User Project:
User Project: Is a request metadata field that is used to assign Storage operational costs to a specified project.

Requester Pays: Is a bucket-level metadata field that is used to enforce the use of the User Project metadata field to assign operational costs when an operation is made on a Bucket and its objects.

[jdpedrie]

Fixed by #694.