Hi
Most of the Dashboards essentially deals with all the monitoring of performance and analysis of the data. So you can start with analysing what type of data is used as the metrics and key performance indicators for your dashboard.
1.Data - The Data can be :MasterData ,Operational Data
Operational data that may deal with values of daily/weekly/monthly.For eg: SalesPerformace dashboard will have metrics for tracking their sales daily,weekly,monthly,quaterly and annual.
Master data which will be remain the same.For eg: Sales area, Region,Product,DeliveryMode/Shipping..
2.KPI& Metrics - Detailed explaination of the Metrics and KPIs
3.Description of the Components used in dashboard
5.When was the data updated and what is the current version . You can also mention about the history of the versions.
6.Any updates in the Key performance indicators
7.Sources for these data : like other softwares like Salesforce or ERP systems
8.Owner of the dashboard
Ideally your documentation should translate the dashboard and provide guidance to the users
Good luck.