Add doc comments

[mappum]

This PR adds basic doc comment coverage to some missing places.

[codecov]

Codecov Report

Attention: Patch coverage is 22.22222% with 7 lines in your changes missing coverage. Please review.

Project coverage is 94.17%. Comparing base (c595a8d) to head (45f3f99).
Report is 5 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/merk/snapshot.rs	0.00%	4 Missing ⚠️
src/merk/mod.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #80      +/-   ##
===========================================
+ Coverage    94.07%   94.17%   +0.10%     
===========================================
  Files           27       27              
  Lines         5449     5443       -6     
===========================================
  Hits          5126     5126              
+ Misses         323      317       -6     

Files with missing lines	Coverage Δ
src/proofs/query/map.rs	98.83% <100.00%> (ø)
src/proofs/tree.rs	94.21% <ø> (ø)
src/test_utils/mod.rs	84.42% <ø> (ø)
src/tree/ops.rs	99.16% <ø> (ø)
src/merk/mod.rs	90.14% <0.00%> (+0.47%)	⬆️
src/merk/snapshot.rs	9.01% <0.00%> (+0.21%)	⬆️