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_MINIRAMBO.h

@@ -107,7 +107,7 @@
     #define BTN_EN2         72
     #define BTN_ENC          9  // the click
 
-    #define SDCARDDETECT_PIN 15
+    #define SD_DETECT_PIN   15
 
   #endif //NEWPANEL
 #endif //ULTRA_LCD