Update docs and changelog

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01% :tada:

Comparison is base (37a2cbd) 97.47% compared to head (ef172bf) 97.48%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #114 +/- ## ========================================== + Coverage 97.47% 97.48% +0.01% ========================================== Files 8 8 Lines 713 717 +4 ========================================== + Hits 695 699 +4 Misses 18 18 ``` | [Flag](https://app.codecov.io/gh/beda-software/fhir-py/pull/114/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=beda-software) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/beda-software/fhir-py/pull/114/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=beda-software) | `97.48% <100.00%> (+0.01%)` | :arrow_up: | 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=beda-software#carryforward-flags-in-the-pull-request-comment) to find out more. | [Files Changed](https://app.codecov.io/gh/beda-software/fhir-py/pull/114?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=beda-software) | Coverage Δ | | |---|---|---| | [fhirpy/base/lib.py](https://app.codecov.io/gh/beda-software/fhir-py/pull/114?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=beda-software#diff-ZmhpcnB5L2Jhc2UvbGliLnB5) | `98.42% <100.00%> (+0.02%)` | :arrow_up: |

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[ir4y]

@pavlushkin information about conditional delete is missing

[pavlushkin]

@ir4y the PR is ready to review. I added conditional delete operations and updated the documentation.