Fix expiry_time issue and add some new documentation

[graham83]

Minor fix to resolve issue https://draftable.atlassian.net/browse/CL-129
Also added some comments to documentation to remind clients to check comparison for ready status

[codecov]

Codecov Report

Base: 75.00% // Head: 75.17% // Increases project coverage by +0.17% 🎉

Coverage data is based on head (544a7d1) compared to base (6238b85).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #3      +/-   ##
==========================================
+ Coverage   75.00%   75.17%   +0.17%     
==========================================
  Files          19       19              
  Lines         576      576              
  Branches       69       53      -16     
==========================================
+ Hits          432      433       +1     
+ Misses        117      116       -1     
  Partials       27       27              

Impacted Files	Coverage Δ
draftable/endpoints/comparisons/comparisons.py	96.15% <100.00%> (ø)
draftable/endpoints/comparisons/comparison.py	65.93% <0.00%> (+1.09%)	⬆️

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[graham83]

@ralish - It was a one liner so went ahead and merged.