[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"
    */

hal/src/main/native/include/hal/HALBase.h

@@ -110,6 +110,10 @@ HAL_RuntimeType HAL_GetRuntimeType(void);
 /**
  * Gets the state of the "USER" button on the roboRIO.
  *
+ * @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.
+ *
  * @param[out] status the error code, or 0 for success
  * @return true if the button is currently pressed down
  */

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

@@ -83,6 +83,10 @@ class RobotController {
   /**
    * Get the state of the "USER" button on the roboRIO.
    *
+   * @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
    */
   static bool GetUserButton();

wpilibj/src/main/java/edu/wpi/first/wpilibj/RobotController.java

@@ -79,6 +79,10 @@ public final class RobotController {
   /**
    * Get 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
    */
   public static boolean getUserButton() {