Add example to docstrings in rolling.apply

[isVoid]

Partially address #7850

Currently docstrings of rolling.apply has no example to show case the usage of the API. This PR adds that and also points out that currently args and kwargs is unsupported.

[codecov]

Codecov Report

Merging #9522 (be31e93) into branch-21.12 (ab4bfaa) will decrease coverage by 0.11%.
The diff coverage is n/a.

@@               Coverage Diff                @@
##           branch-21.12    #9522      +/-   ##
================================================
- Coverage         10.79%   10.67%   -0.12%     
================================================
  Files               116      117       +1     
  Lines             18869    19714     +845     
================================================
+ Hits               2036     2104      +68     
- Misses            16833    17610     +777     

Impacted Files	Coverage Δ
python/dask_cudf/dask_cudf/sorting.py	92.90% <0.00%> (-1.21%)	⬇️
python/cudf/cudf/io/csv.py	0.00% <0.00%> (ø)
python/cudf/cudf/io/hdf.py	0.00% <0.00%> (ø)
python/cudf/cudf/io/orc.py	0.00% <0.00%> (ø)
python/cudf/cudf/__init__.py	0.00% <0.00%> (ø)
python/cudf/cudf/_version.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/abc.py	0.00% <0.00%> (ø)
python/cudf/cudf/api/types.py	0.00% <0.00%> (ø)
python/cudf/cudf/io/dlpack.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/frame.py	0.00% <0.00%> (ø)
... and 64 more

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[isVoid]

@gpucibot merge