Is the [NT Service\SQLSERVERAGENT] a supported type for SQL Server Agent service? Reading and re-reading  MSDN "Select an Account for the SQL Server Agent Service".  

NT Service\SQLSERVERAGENT is not listed in the "Supported Service Account Types" section.
And, if not, what is it installed with SQL Server for?
What could this possibly mean, I wonder? 
How should I better undesrstand it?
NT Service\SQLSERVERAGENT is the name of an account, not an account type.  

The document refers to account types.  As @Jon-Seigel said, it qualifies as a 'Local non-administrative user'.