[C++][Docs] Clarify that not all compute functions are invocable via CallFunction

[asfimport]

It has been noted by users that the Compute Functions documentation ("Invoking functions" section) states that functions are invoked via CallFunction, but there exist functions (specifically ["Grouped aggregations"|#grouped-aggregations-group-by]) that can't be invoked directly via CallFunction. This is a bit misleading, although this is stated in the description of "Grouped aggregations".

Grouped aggregations are stateful functions and at the moment stateful functions are not invocable in such a way.

We should clarify in "Invoking functions" section that not all compute functions are directly invocable via CallFunction.

Reporter: Eduardo Ponce / @edponce
Assignee: Sanjiban Sengupta / @sanjibansg

PRs and other links:

• GitHub Pull Request #12103
• GitHub Pull Request #12104

_{Note: This issue was originally created as ARROW-15269. Please see the migration documentation for further details.}

[asfimport]

David Li / @lidavidm:
Issue resolved by pull request 12104
#12104