Improve secure mode and secure screens documentation (#13501)
Summary of the issue:
As raised in comments on #13488, the technical design overview could be clarified to be more clear and add more information.
Using the serviceDebug parameter to prevent secure mode on secure screens is a more universal solution than patching source code.
Users have been unclear on what secure mode and secure screens are when reading about recent security fixes.
Description of how this pull request fixes the issue:
Improves the documentation based on the discussion on #13488.
Adds definitions of secure mode and secure screens to the user guide.
