Clarify TOTP setup instructions

[masteradhoc]

Fixes #486

What?

The PR intends to improve user guidance on how to setup the TOTP.

Why?

The current setup text is quite basic and doesnt help unexperienced users enough to understand how the setup has to be done.

How?

After many contributors feedback in #486 this text is a draft to discuss.

Testing Instructions

1. go to wp-admin/users.php
2. choose your user and edit it
3. see the TOTP setup text
4. apply the PR
5. check the new TOTP setup text

Screenshots or screencast

Current State

New Proposal

Changelog Entry

Changed - Clarify TOTP setup instructions

[github-actions]

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Co-authored-by: masteradhoc <masteradhoc@git.wordpress.org>
Co-authored-by: pbiron <pbiron@git.wordpress.org>
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