Update docs and changelog

[pavlushkin]

No description provided.

[codecov-commenter]

Codecov Report

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

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

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Additional details and impacted files

@@            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	Coverage Δ
unittests	97.48% <100.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
fhirpy/base/lib.py	98.42% <100.00%> (+0.02%)	⬆️

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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.