Refactor docx_comments. Closes #541.

[trekonom]

This PR refactors docx_comments to (properly) account for comments spanning
multiple paragraphs, nested comments, multiple comments in the same paragraph and replies. Closes #541.

• Adds a test document to check that docx_comments accounts for these "special" cases found in real-world documents.

• The dataframe returned by docx_comments() gains a list column para_id containing
  the id(s) of the parent paragraph(s). A possible breaking change is that the commented_text
  is now returned as a list column to account for comments spanning multiple runs.

• The docs gain a description of the columns of the returned dataframe.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (58f5c52) 72.61% compared to head (9a44f52) 73.24%.
Report is 1 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #544      +/-   ##
==========================================
+ Coverage   72.61%   73.24%   +0.63%     
==========================================
  Files          40       40              
  Lines        5499     5494       -5     
==========================================
+ Hits         3993     4024      +31     
+ Misses       1506     1470      -36     

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[davidgohel]

@trekonom it looks good to me. Thank you very much for the preciously documented and tested work, it's really very pleasant.