ARROW-5612: [Python][Doc] Add prominent note that date_as_object option changed with Arrow 0.13

[mfcabrera]

Adding small documentation on bits on the pandas integration documentation. It relates to #4363
Not sure if the wording is correct.

[codecov-io]

Codecov Report

Merging #4381 into master will decrease coverage by 24.56%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master    #4381       +/-   ##
===========================================
- Coverage   88.34%   63.78%   -24.57%     
===========================================
  Files         781      385      -396     
  Lines       98721    55678    -43043     
  Branches     1251        0     -1251     
===========================================
- Hits        87219    35514    -51705     
- Misses      11266    20164     +8898     
+ Partials      236        0      -236

Impacted Files	Coverage Δ
cpp/src/arrow/util/memory.h	0% <0%> (-100%)	⬇️
cpp/src/parquet/hasher.h	0% <0%> (-100%)	⬇️
cpp/src/arrow/extension_type.h	0% <0%> (-100%)	⬇️
cpp/src/arrow/compute/kernels/compare.h	0% <0%> (-100%)	⬇️
cpp/src/arrow/util/memory.cc	0% <0%> (-100%)	⬇️
cpp/src/arrow/util/sse-util.h	0% <0%> (-100%)	⬇️
cpp/src/arrow/json/chunker.h	0% <0%> (-100%)	⬇️
cpp/src/arrow/compute/logical_type.h	0% <0%> (-100%)	⬇️
cpp/src/arrow/compute/kernels/filter.cc	0% <0%> (-100%)	⬇️
cpp/src/arrow/json/parser.h	0% <0%> (-100%)	⬇️
... and 592 more

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[xhochy]

I would think that these comments should rather go into the pandas documentation not. @jorisvandenbossche @wesm what do you think?

[jorisvandenbossche]

The note about the change in default for dates might be useful here, but I agree that the note about parquet belongs more in the pandas docs.

It might be useful to add a note in the parquet section of the docs (so not the in the pandas section) that when you read the parquet file into a pandas dataframe (either through Table.to_pandas after reading the table or either with pd.read_parquet) the type conversion options are also related to the arrow -> pandas conversion options, and link to the pandas.rst "type differences" section about that.

[wesm]

+1. I scrubbed out the bit which should go in the pandas documentation (@jorisvandenbossche, do you want to pick that up?) and fleshed out the warning about date_as_object in older versions of pyarrow

[jorisvandenbossche]

(and taking up the pandas docs part)