Doctests for UnionBuilder and UnionArray.

[novemberkilo]

Which issue does this PR close?

Closes #301.

(@alamb do you concur that this PR closes the issue. Or have I missed an array type that could use a doctest?)

What changes are included in this PR?

Repurposes existing examples for UnionBuilder and putting them into doctests.
Adds some documentation for UnionArray

Are there any user-facing changes?

No

[codecov-commenter]

Codecov Report

Merging #603 (1d49007) into master (87d2840) will increase coverage by 0.01%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #603      +/-   ##
==========================================
+ Coverage   82.41%   82.42%   +0.01%     
==========================================
  Files         167      167              
  Lines       46232    46257      +25     
==========================================
+ Hits        38100    38126      +26     
+ Misses       8132     8131       -1     

Impacted Files	Coverage Δ
arrow/src/array/array_union.rs	89.26% <ø> (ø)
arrow/src/array/builder.rs	86.26% <50.00%> (+0.15%)	⬆️
arrow/src/array/ord.rs	63.33% <0.00%> (ø)
arrow/src/compute/kernels/substring.rs	98.27% <0.00%> (ø)
arrow/src/compute/kernels/partition.rs	97.65% <0.00%> (+0.15%)	⬆️
parquet/src/encodings/encoding.rs	95.04% <0.00%> (+0.19%)	⬆️

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[alamb]

Thanks @novemberkilo

In terms of completing the documentation, I looked at all the things that implemented Array here: https://docs.rs/arrow/5.0.0/arrow/array/trait.Array.html#implementors

I think https://docs.rs/arrow/5.0.0/arrow/array/type.LargeBinaryArray.html and https://docs.rs/arrow/5.0.0/arrow/array/type.BinaryArray.html are stll missing examples, but othrwise it is looking good.

Thanks so much @novemberkilo

[novemberkilo]

@alamb I will add examples for LargeBinaryArray and BinaryArray to this PR in the next few days.

[alamb]

Great -- Thank you @novemberkilo -- feel free to reopen #301 if you want or just mention it.

Again, I really appreciate your help in this area, and I think future users will as well. 👍