docs: replace calls to __del__ with calls to close

[lafrech]

Related to #190.

Proposed Changes

Use close rather than __del__ in docs, tests and examples.

I could have added a close method to ApiClient for consistency but I didn't.

This SO answer provides a nice explanation of what __del__ is and its limitations and why using it is not a great pattern.

At least, I think the content should be in the close method, and the close method should be called in __del__ rather than the opposite.

Anyway, as said in #190, a context manager would be a nice way of solving the whole thing.

Until then, I figured using close in the docs would be better.

Checklist

• CHANGELOG.md updated
• Rebased/mergeable
• A test has been added if appropriate
• pytest tests completes successfully
• Commit messages are in semantic format
• Sign CLA (if not already signed)

[codecov]

Codecov Report

Merging #200 (5836c6d) into master (a3e18ee) will increase coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #200      +/-   ##
==========================================
+ Coverage   89.88%   89.93%   +0.05%     
==========================================
  Files          26       26              
  Lines        1967     1967              
==========================================
+ Hits         1768     1769       +1     
+ Misses        199      198       -1     

Impacted Files	Coverage Δ
influxdb_client/client/write_api.py	97.58% <0.00%> (+0.48%)	⬆️

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[bednar]

Thanks for your PR 👍

Could you please update a CHANGELOG.md file? Something like:

### Documentation
1. [#200](https://github.com/influxdata/influxdb-client-python/pull/200): Updated docs about `DeleteApi`.

[lafrech]

Done.

[bednar]

Thanks again for your PR 👍

[bednar]

@rhajek will take care about #190