[C++] Improve docstrings in new public APIs in arrow/compute and fix miscellaneous typos

[asfimport]

I've noticed some imprecise language while reading the headers and some other opportunities for improvement

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

Related issues:

• [C++] C++ array kernels framework and execution buildout (umbrella issue) (is a child of)

PRs and other links:

• GitHub Pull Request #7264

_{Note: This issue was originally created as ARROW-8926. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 7264
#7264