Update docs to include metrics buffer changes

[binarylogic]

This is a follow-up issue to represent documentation changes related to #930. Throughout our docs we talk about buffers and how they work:

https://docs.vector.dev/usage/configuration/sinks/aws_cloudwatch_logs#buffers-and-batches

I'd like to update our docs to clarify how it works with metrics. These docs were written with logs in mind.

[bruceg]

@binarylogic Is this still relevant? Does it belong in #6514?

[jszwedko]

Closing this since the docs appear to be agnostic to the data type now, just referring to "events".