Correct ORC docstring; other minor cuIO improvements

[vuule]

Fixes #6923

Included other minor cuIO improvements that are too small for individual PRs:

• Remove unnecessary NaN-related conditions in JSON, CSV.
• Expand a comment in createSerializedTrie to make initialization clearer.

[codecov]

Codecov Report

Merging #7012 (2cc303b) into branch-0.18 (a5515f2) will increase coverage by 0.28%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           branch-0.18    #7012      +/-   ##
===============================================
+ Coverage        82.01%   82.29%   +0.28%     
===============================================
  Files               96       96              
  Lines            16340    16627     +287     
===============================================
+ Hits             13402    13684     +282     
- Misses            2938     2943       +5     

Impacted Files	Coverage Δ
python/cudf/cudf/utils/ioutils.py	78.71% <ø> (ø)
python/dask_cudf/dask_cudf/io/tests/test_csv.py	100.00% <0.00%> (ø)
python/cudf/cudf/core/column/column.py	88.03% <0.00%> (+0.10%)	⬆️
python/dask_cudf/dask_cudf/core.py	74.01% <0.00%> (+0.32%)	⬆️
python/cudf/cudf/core/column/datetime.py	89.42% <0.00%> (+0.53%)	⬆️
python/cudf/cudf/core/indexing.py	96.35% <0.00%> (+0.72%)	⬆️
python/cudf/cudf/core/column/numerical.py	95.88% <0.00%> (+1.31%)	⬆️
python/dask_cudf/dask_cudf/io/csv.py	96.96% <0.00%> (+1.44%)	⬆️
python/cudf/cudf/core/tools/datetimes.py	84.04% <0.00%> (+2.44%)	⬆️

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[galipremsagar]

Python changes LGTM

[galipremsagar]

rerun tests