[doc] Clarify mount orientation for SystemCore IMU (NFC) (#8088)

wpilibc/src/main/native/include/frc/OnboardIMU.h

@@ -22,11 +22,13 @@ class OnboardIMU {
    * A mount orientation of SystemCore
    */
   enum MountOrientation {
-    /** Flat */
+    /** Flat (mounted parallel to the ground). */
     kFlat,
-    /** Landscape */
+    /** Landscape (vertically mounted with long edge of SystemCore parallel to
+       the ground). */
     kLandscape,
-    /** Portrait */
+    /** Portrait (vertically mounted with the short edge of SystemCore parallel
+       to the ground). */
     kPortrait
   };