Further cleanup of comments, partial Doxygen-style

Following up on #3231

Marlin/ultralcd_implementation_hitachi_HD44780.h

@@ -1,4 +1,4 @@
-/*
+/**
  * Marlin 3D Printer Firmware
  * Copyright (C) 2016 MarlinFirmware [https://github.com/MarlinFirmware/Marlin]
  *
@@ -533,7 +533,7 @@ unsigned lcd_print(char c) { return charset_mapper(c); }
 
 #endif // SHOW_BOOTSCREEN
 
-/*
+/**
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