Update and clarify CSV variable example

**Feedback**: The CSV variable "example" isn't helpful at all. What are value1, value2 etc? Names, values? Where are their values? How do you invoke them in a query?

- I think we need to clarify that a CSV variable is simple a comma-delimited list of string values. These values represent all the potential values of whatever variable you're creating.
- We should also include a link to the "Use dashboard variables" section: https://docs.influxdata.com/influxdb/v2.3/visualize-data/variables/#use-dashboard-variables

##### Relevant URLs
- https://docs.influxdata.com/influxdb/cloud/visualize-data/variables/variable-types/

Start with the variable-types documentation at https://docs.influxdata.com/influxdb/cloud/visualize-data/variables/variable-types/ and locate the CSV variable example. Clarify that CSV variables are comma-delimited string values, explain how they are used in queries, and link to the “Use dashboard variables” section; verify the rendered documentation reflects these changes.

Written by the indexing model from the issue text.