[C++][Documentation] List hash aggregate kernels somewhere

[asfimport]

Hash aggregate kernels are not listed in compute.rst with the rest of the functions, presumably because they're not intended to be directly callable. However, once ARROW-12759 goes in, we should find some place to list what aggregations are supported with group by.

Reporter: David Li / @lidavidm
Assignee: David Li / @lidavidm

Related issues:

• [C++] Implement hash_aggregate kernels (umbrella issue) (is a child of)
• [C++][Docs] Add note about tdigest in compute functions docs (is duplicated by)

PRs and other links:

• GitHub Pull Request #10887

_{Note: This issue was originally created as ARROW-13311. Please see the migration documentation for further details.}

[asfimport]

David Li / @lidavidm:
Issue resolved by pull request 10887
#10887