Add reason to unverify work mail

[pixelbrackets]

Why:

Explain the reason why it is a good thing to remove the verified work mail address and then add it again without verifying it.

Otherwise the reader asks himself „why should I do this step? This seems superfluous.“.

Closes #17991

What's being changed:

Add the reason into this paragraph straight away.

Check off the following:

• I have reviewed my changes in staging (look for "Automatically generated comment" and click Modified to view your latest changes).
• For content changes, I have completed the self-review checklist.

Writer impact (This section is for GitHub staff members only):

• This pull request impacts the contribution experience
  • I have added the 'writer impact' label
  • I have added a description and/or a video demo of the changes below (e.g. a "before and after video")

[welcome]

Thanks for opening this pull request! A GitHub docs team member should be by to give feedback soon. In the meantime, please check out the contributing guidelines.

[github-actions]

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This comment is automatically generated and will be overwritten every time changes are committed to this branch.

The table contains an overview of files in the content directory that have been changed in this pull request. It's provided to make it easy to review your changes on the staging site. Please note that changes to the data directory will not show up in this table.

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Content directory changes

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Source	Preview	Production	What Changed
account-and-profile/setting-up-and-managing-your-personal-account-on-github/managing-personal-account-settings/best-practices-for-leaving-your-company.md	fpt ghec	fpt ghec

[janiceilene]

@pixelbrackets Thanks so much for opening a PR! I'll get this triaged for review ⚡