Add missing documentation in aggregation.hpp

[karthikeyann]

Fixes parts of #9373
added missing documentation in aggregation.hpp to fix doxygen warnings
fixes 108 warnings.

[codecov]

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[karthikeyann]

using declaration style here instead of old code.
This is the only code change in this PR.

[davidwendt]

I don't know if the returned std::unique_ptr<whatever> needs to say @return A pointer to a whatever. I'd vote for it just saying @return A whatever object instead.
FWIW, there are some inconsistencies here:
https://github.com/rapidsai/cudf/pull/10887/files#diff-2fb738b709da644f7015e9d81f1aaf571f80b9ff8e33c9fa7634aa8400e9ea2cR670
@return A pointer to a new TDIGEST aggregation
and here:
https://github.com/rapidsai/cudf/pull/10887/files#diff-2fb738b709da644f7015e9d81f1aaf571f80b9ff8e33c9fa7634aa8400e9ea2cR708
@return A MERGE_TDIGEST aggregation object.

[karthikeyann]

I'd vote for it just saying @return A whatever object instead.

I would prefer to remove pointer as well. but object is also not accurate since we don't return object, but unique_ptr. but if the convention is to use object at places where unique_ptr is used, this update is fine. updated few other locations with same update.

[karthikeyann]

rerun tests

[karthikeyann]

rerun tests

[karthikeyann]

rerun tests

[karthikeyann]

rerun tests

[karthikeyann]

@gpucibot merge