[REVIEW] Add isin examples in Docstring

[galipremsagar]

Fully resolves: #7412

Most of the fix to address #7412 were done as part of pandas upgrade in #7375. This PR only includes docstrings update to isin.

[kkraus14]

@gpucibot merge

[codecov]

Codecov Report

Merging #7479 (c42750b) into branch-0.19 (43b44e1) will increase coverage by 0.05%.
The diff coverage is 87.63%.

@@               Coverage Diff               @@
##           branch-0.19    #7479      +/-   ##
===============================================
+ Coverage        81.80%   81.86%   +0.05%     
===============================================
  Files              101      101              
  Lines            16695    16883     +188     
===============================================
+ Hits             13658    13821     +163     
- Misses            3037     3062      +25     

Impacted Files	Coverage Δ
python/cudf/cudf/utils/docutils.py	97.36% <50.00%> (-2.64%)	⬇️
python/cudf/cudf/testing/testing.py	80.00% <57.14%> (-1.04%)	⬇️
python/cudf/cudf/core/column/timedelta.py	88.23% <75.00%> (ø)
python/cudf/cudf/core/column/categorical.py	91.38% <77.14%> (-0.85%)	⬇️
python/cudf/cudf/core/column/datetime.py	89.09% <78.57%> (+0.04%)	⬆️
python/dask_cudf/dask_cudf/core.py	71.67% <79.41%> (-2.60%)	⬇️
python/cudf/cudf/core/multiindex.py	82.28% <90.90%> (+0.12%)	⬆️
python/cudf/cudf/core/tools/datetimes.py	84.44% <93.33%> (+0.80%)	⬆️
python/cudf/cudf/core/column_accessor.py	95.31% <95.65%> (+2.37%)	⬆️
python/cudf/cudf/core/column/column.py	87.76% <96.42%> (+0.34%)	⬆️
... and 21 more

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