[docs] Add warning about using user button to docs (NFC) (#6129)

hal/src/main/java/edu/wpi/first/hal/HALUtil.java

@@ -90,6 +90,10 @@ public final class HALUtil extends JNIWrapper {
   /**
    * Gets the state of the "USER" button on the roboRIO.
    *
+   * <p>Warning: the User Button is used to stop user programs from automatically loading if it is
+   * held for more then 5 seconds. Because of this, it's not recommended to be used by teams for any
+   * other purpose.
+   *
    * @return true if the button is currently pressed down
    * @see "HAL_GetFPGAButton"
    */