plugin options: make time_key description more clear on where timestamp comes from

Signed-off-by: Wesley Pettit <[email protected]>

Author: PettitWesley

README.md

@@ -39,8 +39,8 @@ After this step, run `make windows-release` to build `./bin/firehose.dll`. Then
 * `role_arn`: ARN of an IAM role to assume (for cross account access).
 * `endpoint`: Specify a custom endpoint for the Kinesis Firehose API.
 * `sts_endpoint`: Specify a custom endpoint for the STS API; used to assume your custom role provided with `role_arn`.
-* `time_key`: Add the timestamp to the record under this key. By default the timestamp from Fluent Bit will not be added to records sent to Kinesis.
-* `time_key_format`: [strftime](http://man7.org/linux/man-pages/man3/strftime.3.html) compliant format string for the timestamp; for example, `%Y-%m-%dT%H:%M:%S%z`. This option is used with `time_key`. You can also use `%L` for milliseconds and `%f` for microseconds. If you are using ECS FireLens, make sure you are running Amazon ECS Container Agent v1.42.0 or later, otherwise the timestamps associated with your container logs will only have second precision.
+* `time_key`: Add the timestamp to the record under this key. By default the timestamp from Fluent Bit will not be added to records sent to Kinesis. The timestamp inserted comes from the timestamp that Fluent Bit associates with the log record, which is set by the input that collected it. For example, if you are reading a log file with the [tail input](https://docs.fluentbit.io/manual/pipeline/inputs/tail), then the timestamp for each log line/record can be obtained/parsed by using a Fluent Bit parser on the log line.
+* `time_key_format`: [strftime](http://man7.org/linux/man-pages/man3/strftime.3.html) compliant format string for the timestamp; for example, `%Y-%m-%dT%H:%M:%S%z`. This option is used with `time_key`. You can also use `%L` for milliseconds and `%f` for microseconds. Remember that the `time_key` option only inserts the timestamp Fluent Bit has for each record into the record. So the record must have been collected with a timestamp with precision in order to use sub-second precision formatters. If you are using ECS FireLens, make sure you are running Amazon ECS Container Agent v1.42.0 or later, otherwise the timestamps associated with your stdout & stderr container logs will only have second precision.
 * `replace_dots`: Replace dot characters in key names with the value of this option. For example, if you add `replace_dots _` in your config then all occurrences of `.` will be replaced with an underscore. By default, dots will not be replaced.
 * `simple_aggregation`: Option to allow plugin send multiple log events in the same record if current record not exceed the maximumRecordSize (1 MiB). It joins together as many log records as possible into a single Firehose record and delimits them with newline. It's good to enable if your destination supports aggregation like S3. Default to be `false`, set to `true` to enable this option.
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