Clarify process on broken e2e test that will be introduced to master

A follow up to !18010 (comment 132518168) part of https://gitlab.com/gitlab-org/release/framework/issues/106

• If the change introduces new QA test failures in, package-and-qa or review-qa-all jobs must by run to validate the fix in the same Merge Request.
• Remove confusing clause about making the test change in a separate MR.

@mlapierre @rymai what do you think.

/cc @gl-quality