update doc and variable name for datetime ctor

Description of changes:

Got a customer reach out about the argument being named epoch_millis even thought it was processing the argument as seconds with a milliseconds mantissa. An example of this could be

int64_t timeMillis = 1708552755100;
Utils::DateTime millis(timeMillis);
std::cout << millis.ToGmtString(Utils::DateFormat::ISO_8601) << "\n";
double timeSeconds = 1708552755.100;
Utils::DateTime date(timeSeconds);
std::cout << date.ToGmtString(Utils::DateFormat::ISO_8601) << "\n";

updates the name and comment to make more clear about what the function is doing.

Check all that applies:

[x] Did a review by yourself.
[x] Added proper tests to cover this PR. (If tests are not applicable, explain.)
[x] Checked if this PR is a breaking (APIs have been changed) change.
[x] Checked if this PR will not introduce cross-platform inconsistent behavior.
[x] Checked if this PR would require a ReadMe/Wiki update.

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aws / aws-sdk-cpp

update doc and variable name for datetime ctor #2869