Add some doc comments to parquet bit_util

[alamb]

Rationale for this change

I had to double check a few times what units (bits or bytes) were used for some functions in bit_utils.rs while reviewing #658

What changes are included in this PR?

Encode units in doc comments

[codecov-commenter]

Codecov Report

Merging #663 (65d02af) into master (6bf1988) will decrease coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #663      +/-   ##
==========================================
- Coverage   82.50%   82.42%   -0.08%     
==========================================
  Files         168      168              
  Lines       47249    47250       +1     
==========================================
- Hits        38984    38948      -36     
- Misses       8265     8302      +37     

Impacted Files	Coverage Δ
parquet/src/util/bit_util.rs	93.14% <ø> (ø)
arrow/src/array/transform/boolean.rs	76.92% <0.00%> (-7.70%)	⬇️
arrow/src/array/transform/utils.rs	95.00% <0.00%> (-5.00%)	⬇️
arrow/src/array/equal_json.rs	85.21% <0.00%> (-3.48%)	⬇️
arrow/src/tensor.rs	85.00% <0.00%> (-2.50%)	⬇️
parquet/src/column/page.rs	97.36% <0.00%> (-1.32%)	⬇️
arrow/src/array/equal/utils.rs	74.00% <0.00%> (-1.00%)	⬇️
parquet/src/record/api.rs	91.60% <0.00%> (-0.88%)	⬇️
parquet/src/file/statistics.rs	93.80% <0.00%> (-0.83%)	⬇️
arrow/src/csv/writer.rs	82.41% <0.00%> (-0.74%)	⬇️
... and 16 more

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