docs: document shutdown function

[codecov[bot]]

Codecov Report

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

Comparison is base (e24a634) 93.45% compared to head (5b99041) 93.45%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #237 +/- ## ======================================= Coverage 93.45% 93.45% ======================================= Files 16 16 Lines 443 443 ======================================= Hits 414 414 Misses 29 29 ``` | [Flag](https://app.codecov.io/gh/open-feature/python-sdk/pull/237/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-feature) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/open-feature/python-sdk/pull/237/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-feature) | `93.45% <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=open-feature#carryforward-flags-in-the-pull-request-comment) to find out more.

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