Update documntation for getCardId

Test: manual and atest TelephonyManagerTest
Bug: 131423779
Change-Id: I7b384ec7265e219aee82164476d7d2d96e0f94a0
(cherry picked from commit c43c6b505d)

telephony/java/android/telephony/UiccSlotInfo.java

@@ -140,6 +140,10 @@ public class UiccSlotInfo implements Parcelable {
         return mIsEuicc;
     }
 
+    /**
+     * Returns the ICCID of a the UICC in the given slot, or the EID if it is an eUICC. Note that if
+     * the value is unavailble this will return null.
+     */
     public String getCardId() {
         return mCardId;
     }