Document a good portion of the code

[jwodder]

Part of #32.

[codecov]

Codecov Report

Attention: Patch coverage is 13.79310% with 150 lines in your changes missing coverage. Please review.

Project coverage is 47.08%. Comparing base (e9be2dd) to head (c0d1875).

Files	Patch %	Lines
src/dandi/mod.rs	0.00%	117 Missing ⚠️
src/dav/html.rs	41.17%	10 Missing ⚠️
src/dav/types.rs	0.00%	10 Missing ⚠️
src/dav/mod.rs	0.00%	9 Missing ⚠️
src/dandi/types.rs	0.00%	2 Missing ⚠️
src/dandi/streams.rs	0.00%	1 Missing ⚠️
src/httputil.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #167      +/-   ##
==========================================
+ Coverage   46.29%   47.08%   +0.78%     
==========================================
  Files          27       27              
  Lines        3834     3859      +25     
==========================================
+ Hits         1775     1817      +42     
+ Misses       2059     2042      -17     

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