Added estimation considerations to auth group page

Why is this change being made?

In conversation with @eduardosanz when working on Add a 'Remember me' checkbox to the SAML author... (gitlab-org/gitlab#379013 - closed) we proposed having a few consideration examples in our estimation that may help with more accurate estimates and work breakdown early on during milestone planning. The reviewers (& the team) is welcome to add any other areas that would be beneficial to think about when estimating. The list isn't intended to be exhaustive however.

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
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• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
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• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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