Improve JSON reader documentation

[alamb]

Which issue does this PR close?

re #1538

Rationale for this change

I found @tustvold 's comment helpful #1539 (review) and it wasn't 100% clear to me previously why there was a split between Decoder and Reader. While it was fresh on my mind, I tried to update the doc strings to encode this knowledge for the future

What changes are included in this PR?

Improve doc comments

Are there any user-facing changes?

Better (hopefully) doc strings

[tustvold]

I think you need [``] for the doc link to work correctly

[alamb]

I think you need [``] for the doc link to work correctly

They worked fine for me locally without ``:

Putting them in back ticks does make them look nicer

[tustvold]

TIL, I had always assumed you needed both as that is all I'd ever seen 😅

[codecov-commenter]

Codecov Report

Merging #1559 (9b43297) into master (228ee36) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1559      +/-   ##
==========================================
- Coverage   82.84%   82.84%   -0.01%     
==========================================
  Files         190      190              
  Lines       54985    54985              
==========================================
- Hits        45551    45550       -1     
- Misses       9434     9435       +1     

Impacted Files	Coverage Δ
arrow/src/json/reader.rs	84.48% <ø> (ø)
parquet_derive/src/parquet_field.rs	65.98% <0.00%> (-0.23%)	⬇️
arrow/src/array/transform/mod.rs	86.46% <0.00%> (ø)

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