Improve docstrings + examples for as_primitive_array cast functions

[alamb]

Which issue does this PR close?

N/A

Rationale for this change

I still see people use the more verbose as_any().downcast_ref::<Type>().unwrap code; I also used this frequently in my early arrow days, so I though highlighting and improving the docs for the convenience typecasts might help

What changes are included in this PR?

Add some docstrings and examples

Are there any user-facing changes?

More docs

[codecov-commenter]

Codecov Report

Merging #2114 (e892729) into master (db91031) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2114      +/-   ##
==========================================
- Coverage   83.71%   83.71%   -0.01%     
==========================================
  Files         225      225              
  Lines       59563    59567       +4     
==========================================
+ Hits        49862    49864       +2     
- Misses       9701     9703       +2     

Impacted Files	Coverage Δ
arrow/src/array/mod.rs	100.00% <ø> (ø)
arrow/src/array/cast.rs	100.00% <100.00%> (ø)
arrow/src/datatypes/datatype.rs	62.88% <0.00%> (-0.35%)	⬇️
parquet_derive/src/parquet_field.rs	65.75% <0.00%> (-0.23%)	⬇️
parquet/src/encodings/encoding.rs	93.43% <0.00%> (-0.20%)	⬇️
...row/src/array/builder/string_dictionary_builder.rs	91.36% <0.00%> (+0.71%)	⬆️

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[tustvold]

Merging as clippy failure unrelated to this PR

[ursabot]

Benchmark runs are scheduled for baseline = db91031 and contender = f62f26a. f62f26a is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on ec2-t3-xlarge-us-east-2] ec2-t3-xlarge-us-east-2
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on test-mac-arm] test-mac-arm
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on ursa-i9-9960x] ursa-i9-9960x
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on ursa-thinkcentre-m75q] ursa-thinkcentre-m75q
Buildkite builds:
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python, R. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java