Update docs for better handling of multiple cookies

What does this MR do and why?

Corrects some documentation to prevent malformed cookies being passed to gitlab-qa

How to set up and validate locally

Add an additional export:

$ export QA_COOKIES=gitlab_canary=true

or

$ export QA_COOKIES=gitlab_canary=true\;other_cookie=value

Then follow the instructions later in the same page:

https://gitlab.com/gitlab-org/gitlab-qa/-/blob/master/docs/what_tests_can_be_run.md#testinstancestaging

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.