[dbnode] Update WriteShardsInitializing comment

[ChrisChinchilla]

The comment for WriteShardsInitializing was a duplicate of the comment for ShardsLeavingCountTowardsConsistency, this PR adds a relevant comment.

[codecov]

Codecov Report

Merging #3205 (ee6aa22) into master (0ab7b10) will increase coverage by 14.8%.
The diff coverage is n/a.

❗ Current head ee6aa22 differs from pull request most recent head a42eac0. Consider uploading reports for the commit a42eac0 to get more accurate results

@@            Coverage Diff             @@
##           master    #3205      +/-   ##
==========================================
+ Coverage    57.5%    72.3%   +14.8%     
==========================================
  Files         551     1094     +543     
  Lines       64031   100962   +36931     
==========================================
+ Hits        36824    73032   +36208     
+ Misses      24012    22865    -1147     
- Partials     3195     5065    +1870     

Flag	Coverage Δ
aggregator	75.8% <ø> (+18.6%)	⬆️
cluster	84.9% <ø> (∅)
collector	84.3% <ø> (+25.8%)	⬆️
dbnode	78.7% <ø> (+16.3%)	⬆️
m3em	74.4% <ø> (+27.9%)	⬆️
m3ninx	73.3% <ø> (?)
metrics	20.0% <ø> (+0.2%)	⬆️
msg	74.2% <ø> (-0.2%)	⬇️
query	67.2% <ø> (?)
x	80.1% <ø> (?)

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