Fix doxygen for enum types in libcudf

[davidwendt]

Fix some doxygen formatting errors around enum types found when looking at various pages in the published docs: https://docs.rapids.ai/api/libcudf/stable/namespacecudf.html

[karthikeyann]

LGTM 👍

[codecov]

Codecov Report

Merging #9724 (437f880) into branch-22.02 (967a333) will not change coverage.
The diff coverage is n/a.

@@              Coverage Diff              @@
##           branch-22.02    #9724   +/-   ##
=============================================
  Coverage         10.49%   10.49%           
=============================================
  Files               119      119           
  Lines             20305    20305           
=============================================
  Hits               2130     2130           
  Misses            18175    18175           

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[davidwendt]

@gpucibot merge