Update generated header of unit test classes

Make the purpose of this generated code more clear, and note how this is only generated once.

Author: angularsen

UnitsNet/Scripts/Include-GenerateUnitTestPlaceholderSourceCode.ps1

@@ -4,11 +4,14 @@ function GenerateUnitTestPlaceholderSourceCode($unitClass)
 @"
 //------------------------------------------------------------------------------
 // <auto-generated>
-//     This code was generated by \GenerateUnits.bat.
-//
-//     Changes to this file will be lost when the code is regenerated.
-//     The build server regenerates the code before each build and a pre-build
-//     step will regenerate the code on each local build.
+//     This code was generated (once) by \GenerateUnits.bat, but will not be
+//     regenerated when it already exists. The purpose of creating this file is to make 
+//     it easier to remember to implement all the unit conversion test cases.
+// 
+//     Whenever a new unit is added to this unit class and \GenerateUnits.bat is run,
+//     the base test class will get a new abstract property and cause a compile error
+//     in this derived class, reminding the developer to implement the test case
+//     for the new unit.
 //
 //     See https://github.com/anjdreas/UnitsNet/wiki/Adding-a-New-Unit for how to add or edit units.
 //
@@ -51,4 +54,4 @@ namespace UnitsNet.Tests.CustomCode
     }
 }
 "@;
-}
+}