Improved API docs for PrimitiveArray and Utf8Array

[jorgecarleitao]

This moves code around, adds examples, and improves docs, to make docs easier to follow.

[codecov]

Codecov Report

Merging #1017 (f441847) into main (bbe7209) will increase coverage by 0.15%.
The diff coverage is 86.61%.

@@            Coverage Diff             @@
##             main    #1017      +/-   ##
==========================================
+ Coverage   71.68%   71.84%   +0.15%     
==========================================
  Files         359      359              
  Lines       19860    20192     +332     
==========================================
+ Hits        14237    14507     +270     
- Misses       5623     5685      +62     

Impacted Files	Coverage Δ
src/array/binary/mod.rs	84.46% <ø> (ø)
src/array/equal/utf8.rs	100.00% <ø> (ø)
src/array/fixed_size_binary/mod.rs	88.34% <ø> (ø)
src/array/primitive/from_natural.rs	100.00% <ø> (+20.00%)	⬆️
src/array/primitive/iterator.rs	42.85% <ø> (-17.15%)	⬇️
src/array/utf8/from.rs	100.00% <ø> (ø)
src/array/utf8/iterator.rs	41.66% <ø> (-10.06%)	⬇️
src/io/ipc/write/file_async.rs	75.29% <ø> (ø)
src/io/ipc/write/stream_async.rs	72.41% <ø> (ø)
src/types/bit_chunk.rs	100.00% <ø> (ø)
... and 9 more

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