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/pins_AZTEEG_X3_PRO.h

@@ -109,7 +109,7 @@
   #define BTN_ENC          39  //the click switch
  
   #define SDSS             53
-  #define SDCARDDETECT_PIN 49
+  #define SD_DETECT_PIN 49
   
   #define KILL_PIN         31
  #endif