docs: clarify commit author is determined from local git config

Created by: courier-new

Another piece of the PAT docs update that I should have double-checked was the language around commit authorship, which it turns out we don't use the PAT for but rather take from the git config of the local environment. In retrospect, that does make more sense knowing how src and the batch spec execution works, but I also think it's easy to assume commit author should match changeset author if you're the only person creating a batch change.

Since there's been some confusion around this with at least one customer, here's me taking a stab at removing the erroneous suggestions and making this extra explicit in our docs. 😅