Add context for pricing vectors in data-usage page

A Cloud2 prospect emailed us asking what "Data In" meant more specifically, and pointed to [this page](
https://github.com/influxdata/docs-v2/blob/master/content/influxdb/cloud/account-management/data-usage.md) saying it didn't help answer this question.

_Describe your proposal here._
I propose simply adding a line about how the byte size of metrics is the size of the textual representation of metrics in Line Protocol format. Maybe an example CPU metric with its byte size. Perhaps if we were to choose a more verbose explanation of this (like with example methods of calculating this), it would go somewhere else.