Fix docstring section headings.

[bdice]

Standardizes docstring section headings to reduce parsing warnings in Sphinx output. (I have a few other incoming PRs to fix documentation issues, this one is split into its own PR since it touches a lot of files.)

[codecov]

Codecov Report

Merging #10639 (04d9e24) into branch-22.06 (bf4ffc9) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@               Coverage Diff                @@
##           branch-22.06   #10639      +/-   ##
================================================
+ Coverage         86.33%   86.36%   +0.02%     
================================================
  Files               140      140              
  Lines             22289    22289              
================================================
+ Hits              19244    19250       +6     
+ Misses             3045     3039       -6     

Impacted Files	Coverage Δ
python/cudf/cudf/core/algorithms.py	90.47% <ø> (ø)
python/cudf/cudf/core/dataframe.py	93.59% <ø> (ø)
python/cudf/cudf/core/groupby/groupby.py	91.72% <ø> (+0.22%)	⬆️
python/cudf/cudf/core/multiindex.py	92.14% <ø> (ø)
python/cudf/cudf/core/series.py	95.28% <ø> (ø)
python/cudf/cudf/core/single_column_frame.py	96.45% <ø> (ø)
python/cudf/cudf/utils/cudautils.py	59.83% <ø> (ø)
python/cudf/cudf/utils/utils.py	90.28% <ø> (ø)
python/dask_cudf/dask_cudf/backends.py	86.44% <100.00%> (ø)
python/dask_cudf/dask_cudf/sorting.py	93.03% <100.00%> (ø)
... and 4 more

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[galipremsagar]

@gpucibot merge