Document methods on Rows allowed to be called after Close()

rows.go

@@ -16,6 +16,8 @@ import (
 // the *Conn can be used again. Rows are closed by explicitly calling Close(),
 // calling Next() until it returns false, or when a fatal error occurs.
 //
+// Once a Rows is closed the only methods that may be called are Close(), Err(), and CommandTag().
+//
 // Rows is an interface instead of a struct to allow tests to mock Query. However,
 // adding a method to an interface is technically a breaking change. Because of this
 // the Rows interface is partially excluded from semantic version requirements.