message: Add a line to clarify the 'debug' device exception.
diff: @@ -194,7 +194,8 @@ public:
  * The available backends are described by the Backend class.
  * The string name of a backend is the lowercase version of the
  Backend::Id enumeration constant with '_' replaced by '-', e.g. the
  - string name of 'RAJA_CPU' is 'raja-cpu'.
  + string name of 'RAJA_CPU' is 'raja-cpu'. The string name of the debug
  + backend (Backend::Id 'DEBUG_DEVICE') is exceptionally set to 'debug'.
  * The 'cpu' backend is always enabled with lowest priority.
  * The current backend priority from highest to lowest is:
  'ceed-cuda', 'occa-cuda', 'raja-cuda', 'cuda', 'hip', 'debug',
  