Add symbol col example to scatter visualization docs

Page: https://docs.influxdata.com/influxdb/cloud/visualize-data/visualization-types/scatter/
Feedback: Can you add an example of using the Symbol column, for instance to use the same symbol for all points or use a different symbol for ranges.

Open the scatter visualization page linked in the issue and review how its current examples describe chart columns. Add an example showing the Symbol column used for one symbol across all points and another using different symbols for ranges. Done means the page clearly documents both uses with valid, understandable examples.

Written by the indexing model from the issue text.