docs(readme): explain behavior of unset `auth_tokens` and `delete_tokens`

Description

Just a clarification on how this works in the case of uninstalled tokens, for greater clarity and security

Motivation and Context

Discussed as part of #199, just to clarify the behavior

Changelog Entry

Types of Changes

[ ] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to change)
[X] Documentation (no code change)
[ ] Refactor (refactoring production code)
[ ] Other

Checklist:

[ ] My code follows the code style of this project.
[ ] I have updated the documentation accordingly.
[ ] I have formatted the code with rustfmt.
[ ] I checked the lints with clippy.
[ ] I have added tests to cover my changes.
[ ] All new and existing tests passed.

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (adbd67a) 70.23% compared to head (4b558b5) 70.23%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #202 +/- ## ======================================= Coverage 70.23% 70.23% ======================================= Files 11 11 Lines 598 598 ======================================= Hits 420 420 Misses 178 178 ``` | [Flag](https://app.codecov.io/gh/orhun/rustypaste/pull/202/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Orhun+Parmaks%C4%B1z) | Coverage Δ | | |---|---|---| | [unit-tests](https://app.codecov.io/gh/orhun/rustypaste/pull/202/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Orhun+Parmaks%C4%B1z) | `70.23% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Orhun+Parmaks%C4%B1z#carryforward-flags-in-the-pull-request-comment) to find out more.

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orhun / rustypaste