Update insert-functions.md to correct onBatch example

[thumbsupep]

Proposed changes

per a support request between the company I work for and Segment support, Segment's engineers were able to repro a bug with the onBatch handler, and recommended that consumers preserve the ordering in which events are received as a workaround. I have copied their explanation and code sample verbatim.

This correction of the sample code documentation will help consumers and should save some bandwidth for the Segment support team.

Merge timing

ASAP once approved

[forstisabella]

Suggested change

	**However**, Segment's function invoker service relies on positional consistency between the input and output arrays when using the `onBatch` handler. When a function returns a transformed batch, Segment pairs each output event with its corresponding input using array index positions—not event IDs or timestamps. Consumers must preserve the original order in their `onBatch` implementation.
	You must preserve the original order in the `onBatch` implementation, as Segment's function invoker service relies on positional consistency in the `onBatch` handler between the input and output arrays. When a function returns a transformed batch, Segment pairs each output event with its corresponding input using array index positions, not event IDs or timestamps.