[C++][Compute] Strftime kernel follow-up

[asfimport]

As per ARROW-13174 comments we should:

• Correct default format string for non-UTC timestamps
• Allow non-zoned timestamps to be printed
• Better document %S flag behavior

Reporter: Rok Mihevc / @rok
Assignee: Rok Mihevc / @rok
Watchers: Rok Mihevc / @rok

PRs and other links:

• GitHub Pull Request #10998

_{Note: This issue was originally created as ARROW-13684. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
Issue resolved by pull request 10998
#10998