SDCARDDETECT -> SD_DETECT, added docs

- `SD_DETECT_PIN` replaces `SDCARDDETECT`
- `SD_DETECT_INVERTED` replaces `SDCARDDETECTINVERTED`
- Revise the description of `SD_DETECT_INVERTED`
- Add a note about the override of `SD_DETECT_INVERTED` in
`Conditionals.h`

Marlin/Configuration.h

@@ -674,7 +674,7 @@ const bool Z_PROBE_ENDSTOP_INVERTING = false; // set to true to invert the logic
 //
 // ==> REMEMBER TO INSTALL U8glib to your ARDUINO library folder: http://code.google.com/p/u8glib/wiki/u8glib
 //#define ELB_FULL_GRAPHIC_CONTROLLER
-//#define SDCARDDETECTINVERTED
+//#define SD_DETECT_INVERTED
 
 // The RepRapDiscount Smart Controller (white PCB)
 // http://reprap.org/wiki/RepRapDiscount_Smart_Controller