Allow control over Enum repr

[rhshadrach]

Is your feature request related to a problem? Please describe.

From #9272, this commit removed any user control over the representation of the Enum in the docs. As a result, what we would like to be represented as <no_default> becomes the slightly ugly _NoDefault.no_default in the pandas docs.

Describe the solution you'd like
Add a check to see if the user is overwriting Enum's repr: object.__repr__ is Enum.__repr__. If they are overwriting it, just use the repr as-is.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
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[picnixz]

PR is welcomed. I agree that using a custom __repr__ (if available) is better.