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_RAMPS_13.h

@@ -165,25 +165,25 @@
       #define BTN_EN2 33
       #define BTN_ENC 35
 
-      #define SDCARDDETECT_PIN 49
+      #define SD_DETECT_PIN 49
     #elif ENABLED(LCD_I2C_PANELOLU2)
       #define BTN_EN1 47  // reverse if the encoder turns the wrong way.
       #define BTN_EN2 43
       #define BTN_ENC 32
       #define LCD_SDSS 53
-      #define SDCARDDETECT_PIN -1
+      #define SD_DETECT_PIN -1
       #define KILL_PIN 41
     #elif ENABLED(LCD_I2C_VIKI)
       #define BTN_EN1 22  // reverse if the encoder turns the wrong way.
       #define BTN_EN2 7
       #define BTN_ENC -1
       #define LCD_SDSS 53
-      #define SDCARDDETECT_PIN 49
+      #define SD_DETECT_PIN 49
     #elif ENABLED(ELB_FULL_GRAPHIC_CONTROLLER)
       #define BTN_EN1 35  // reverse if the encoder turns the wrong way.
       #define BTN_EN2 37
       #define BTN_ENC 31
-      #define SDCARDDETECT_PIN 49
+      #define SD_DETECT_PIN 49
       #define LCD_SDSS 53
       #define KILL_PIN 41
       #define BEEPER_PIN 23
@@ -210,7 +210,7 @@
        #define BTN_EN2 63
        #define BTN_ENC 59  //the click switch
        //not connected to a pin
-       #define SDCARDDETECT_PIN 49
+       #define SD_DETECT_PIN 49
 
     #else
 
@@ -235,9 +235,9 @@
       #endif
 
       #if ENABLED(G3D_PANEL)
-        #define SDCARDDETECT_PIN 49
+        #define SD_DETECT_PIN 49
       #else
-        #define SDCARDDETECT_PIN -1  // Ramps doesn't use this
+        #define SD_DETECT_PIN -1  // Ramps doesn't use this
       #endif
 
     #endif