[C++][Docs] Add note about tdigest in compute functions docs

[asfimport]

Users who are looking for an approximate quantile kernel might not recognize that tdigest is this. Add a note in docs/source/cpp/compute.rst saying that tdigest outputs the approximate quantile.

Reporter: Ian Cook / @ianmcook
Assignee: David Li / @lidavidm

Related issues:

• [C++][Documentation] List hash aggregate kernels somewhere (duplicates)

_{Note: This issue was originally created as ARROW-13521. Please see the migration documentation for further details.}

[asfimport]

Ian Cook / @ianmcook:
There is a fix for this in #10887