I have a saved search that uses "stats count by xyz" which produces a simple table search result. Is there any way I can add a legend or text block to the search result to clarify the column names? I want to add text that says "response time is in milliseconds and throughput is requests/second".
You should be able to customize charts, tables, and other visualizations in dashboards and dashboard panels via the GUI or Simple XML customizations:
http://docs.splunk.com/Documentation/Splunk/latest/Viz/Chartcustomization
Your specific needs may be met by simple changes, or you may need to use deeper properties:
http://docs.splunk.com/Documentation/Splunk/latest/Viz/CustomChartingConfig-ChartLegend
While it is also possible to use Advanced XML, custom modules, the SDKs, or the API, your probably don't need that.
You could change the column titles by renaming the fields, such as this:
... | rename response_time as "Response Time (sec)" throughput as "Throughput (reqs/sec)"