Note (MarlinUI) limit on PREHEAT settings (#20966)

Co-authored-by: Scott Lahteine <thinkyhead@users.noreply.github.com>

Marlin/Configuration.h

 
 // @section temperature
 
-// Preheat Constants
+//
+// Preheat Constants - Up to 5 are supported without changes
+//
 #define PREHEAT_1_LABEL       "PLA"
 #define PREHEAT_1_TEMP_HOTEND 180
 #define PREHEAT_1_TEMP_BED     70