[C++][Docs] Clarify expectation of UTC values for timestamps with time zones in C++ API docs

[asfimport]

For timestamp datatypes, if the timezone parameter is non-empty, the int64 array values in the associated column are assumed to be normalized to UTC.  This requirement should be made clear to the C++ API user.  (It can be inferred from the flatbuffers schema, but that internal implementation document probably wouldn't ordinarily be consulted by a C++ API consumer.)

Reporter: TP Boudreau / @tpboudreau
Assignee: TP Boudreau / @tpboudreau

PRs and other links:

• GitHub Pull Request #4555

_{Note: This issue was originally created as ARROW-5545. Please see the migration documentation for further details.}

[asfimport]

TP Boudreau / @tpboudreau:
On further review, the flatbuffers spec contains quite a bit of usage information and probably is intended to be a primary resource for developers.  (Sorry for the noise.)

Closing this issue.

[asfimport]

Wes McKinney / @wesm:
Reopening. I think the C++ header files should make it clear what is expected, this seems like an opportunity to improve them

[asfimport]

TP Boudreau / @tpboudreau:
OK, I've re-assigned to myself.

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 4555
#4555