update doc of array_binary and array_string

[HaoYang670]

Signed-off-by: remzi 13716567376yh@gmail.com

Which issue does this PR close?

close #1452
close #1453
close #1454

What changes are included in this PR?

update doc.

[codecov-commenter]

Codecov Report

Merging #1491 (7da5769) into master (6bf3b3a) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1491   +/-   ##
=======================================
  Coverage   82.72%   82.73%           
=======================================
  Files         188      188           
  Lines       54286    54286           
=======================================
+ Hits        44908    44912    +4     
+ Misses       9378     9374    -4     

Impacted Files	Coverage Δ
arrow/src/array/array_binary.rs	92.62% <ø> (ø)
arrow/src/array/array_string.rs	97.65% <ø> (ø)
arrow/src/array/transform/mod.rs	86.46% <0.00%> (+0.11%)	⬆️
arrow/src/datatypes/datatype.rs	66.80% <0.00%> (+0.39%)	⬆️
parquet_derive/src/parquet_field.rs	66.21% <0.00%> (+0.45%)	⬆️

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